The Under-1cc FROG Motors

by Adrian Duncan

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    The 1/2 cc diesel revolution of 1951/52
    1952—Forerunner of the Frog 80—the Frog 50
    1957—The Frog 80 Mk I diesel appears
    The Frog 80 Mk I—description
    1959—the British glow-plug revolution
    1959—The Frog .049
    The Frog .049 On Test
    The Frog .049 in the Marketplace—A Personal Recollection
    1961—The Frog 80 Mk II
    1962/64—A change of Manufacturer
    1964—The Frog 80 Mk III
    End of the Road

In earlier articles, we've looked at two of the contenders in the British 1/2 A "glow-plug revolution" which began in mid 1959 and continued for a few years thereafter before most of the participating models faded from the scene, leaving the Davies-Charlton Bantam alone holding the field. To extend our in-depth coverage of that particular episode in British model engine history, it seemed only right that we should next turn our attention to the engine whose introduction actually triggered that contest—the Frog .049.

However, it proved to be impossible to treat the Frog .049 in isolation, since its story is inextricably entwined with that of its diesel progenitor, the Frog 80, the seeds of which were themselves planted some years previously. The Frog .049 was in effect simply a glow-plug conversion of the Frog 80, albeit with a few distinctive design features, and its design in turn greatly influenced that of the later diesel models. Accordingly, to review it in isolation would require us to ignore its proper place in the history of the Frog range in general. No-one would learn very much from such an exercise!

So in order to make sense of the Frog .049 story, it's necessary to review the history of the Frog under-1 cc models in general—to do otherwise would be to misrepresent the context in which the Frog .049 came into being. We therefore have to go back over a decade from 1959 to pick up the thread which led to the successive creation of the Frog 50 and 80 diesels and subsequently the Frog .049.

But before we do so, I'd like to record my grateful acknowledgement of the considerable assistance rendered to me by my good mate and valued colleague Kevin Richards. Kevin is best known as the world's leading authority on the ED range, but he knows a thing or two about Frogs as well! The value of the contribution that he made to this study in terms of both data and commentary cannot be overstated—many thanks!

Quite apart from his usual outstanding editorial efforts, I'm also extremely grateful to our friendly Editor Ron Chernich for taking time from his busy schedule to dig out a few tests and advertisements that I didn't have. Only help like this makes it possible to claim any sort of authority for these articles, and Ron's contribution is of immense value in that regard.


The model diesel (or more correctly, compression ignition) engine was a wartime European development, although the hostilities which engulfed Europe during those years prevented much coordinated development from taking place. However, the conclusion of the War created conditions under which the pace of model engine development was greatly accelerated. One of the British firms which took a leading part in this early post-war activity was International Model Aircraft (IMA).

IMA was one of the oldest-established model aircraft manufacturers in the world, having been founded in 1931 by Charles Wilmot and Joe Mansour. The trade-name "FROG" ("Flies Right Off Ground") was used by IMA from the outset for their popular flying model kits and was later carried over to their engines. A detailed and well illustrated history of FROG and all its products appears in the hardcover book, FROG Model Aircraft 1932-1976.

In early 1932 IMA became part of the very prolific Lines Brothers organization, which had been a major player in the British toy industry since 1919, continuing a Lines family involvement in that industry which had begun in around 1850. Lines Brothers also owned the famous Tri-ang toy brand, the triangle emblem of which represented the three Lines brothers who owned the company (three Lines make a triangle!). Arthur Lines became the Chairman of IMA at this point, but Wilmot and Mansour both remained on board as Directors.

During WW2, the Lines Brothers organization was understandably preoccupied with the production of military materiel, notably the iconoclastic Sten gun as well as other military hardware. However, the end of the conflict triggered a general desire for a rapid return to normalcy, and Lines Brothers quickly resumed their former activities, including the manufacture of model products by their IMA subsidiary.

Although Lines Brothers marketed a wide variety of toy and model products through their various subsidiaries, the bulk of their early post-war manufacturing for all their brands was concentrated at their very large factory located on a 27 acre site at Morden Road (today's A24) in the south-west London suburb of Merton in Surrey. The exact location is on the east side of Morden Road just to the south of its junction with today's Merantum Way and a little north of Morden Road station.

This facility had been established in 1925 and was known as Tri-ang Works. By the time of which we are speaking it was claimed to be the largest toy and model production facility in the world, occupying some 750,000 square feet of floor space and employing over 4,000 people. It generated enough business to justify its own half-mile rail siding. IMA's production facilities occupied only a modest portion of this giant complex.

As a result of the importance of their wartime work making military hardware, Lines Brothers had enjoyed a high priority in terms of keeping their plant and equipment up to date and in good condition throughout the conflict. The factory had been damaged by enemy action during the war, but was quickly refurbished. Consequently, the company entered the post-war period with an intact factory and plenty of good quality machine tools on hand—no need to go scrounging for clapped-out wartime surplus equipment like so many others.

Using these excellent facilities, the manufacture of model engines by IMA soon commenced at Morden Road, the first such product being the 1.75 cc spark ignition motor which appeared in early 1946. This was soon followed by IMA's first diesel, the Frog 100, which made its initial appearance in pre-production form later in 1946 and reached the market in quantity in early 1947. From that point onwards, diesels quickly took over from spark ignition engines as the preferred choice of modellers in Britain and the rest of Europe, and even the Americans took a hand in the game with several excellent model diesel designs of their own.

The advent of the glow-plug in commercial form in the USA during early 1948 soon diverted the vast majority of US modellers away from diesels towards the new form of ignition. However, this switch was by no means echoed in Europe, where modellers continued to prefer diesels for all but a few specialized applications such as all-out racing and large-displacement flying.

One of the clear advantages of the model diesel engine was its potential for construction in near-microscopic sizes. A succession of progressively-smaller commercial diesels appeared in Britain during the latter half of the 1940's, culminating in the early 1948 appearance of the 0.2 cc Kemp (later K) Hawk. The talented Swedish model engineer Harry Fjellström had gone even further than this, setting the bench-mark in 1947 by producing a successful one-off model diesel having a displacement of only 0.04 cc (.0024 cu. in.). This little masterpiece was demonstrated in public on many occasions, to the point where it eventually had to be rebored!

The 1/2 cc Diesel Revolution of 1951/52

Several of the sub-miniature diesels from the period just discussed achieved a reasonable degree of commercial success, notably the aforementioned Kemp Hawk, of which more elsewhere. However, it can't be denied that the output of these miniatures was a bit marginal for practical purposes, especially in the control-line field which was then becoming increasingly dominant in British aeromodelling.

Despite this, there was certainly a market for small engines in Britain, where flying fields of adequate area were relatively few and far between in and around major population centres and noise was even then an issue for urban modellers. So the challenge facing British manufacturers was to develop engines which produced substantially more power than their sub-miniature predecessors while at the same time remaining sufficiently compact and light to be useable in very small models.

The first designer to come up with a really effective response to this challenge was Alan Allbon, who had been in business since 1947 under the company name of Allbon Engineering from a workshop at 51A Thames Street, Sunbury-on-Thames, Surrey. In 1950, Allbon developed perhaps his most famous and enduring design in the shape of the 0.55 cc Dart diesel. Dennis Allen of later Allen-Mercury fame was working with Allbon at this time and probably played a significant role in the development of the Dart.

This neat little engine met all of the requirements set out above—it was compact, very light and remarkably powerful for its displacement. Despite a selling price of £3 5s 0d (£3.25 in new money), which was a fair chunk of change in 1950 Britain, the marketplace was more than ready for the Dart when it appeared in late 1950, and demand soon outstripped Allbon's ability to manufacture it. This eventually forced him into a 1952 merger with Davies-Charlton (D-C) Ltd, as related in The DC Story.

Allbon's immediate success with the Dart was by no means lost upon others, and no fewer than three other competing manufacturers (Elfin, IMA and ED) quickly followed suit with 1/2 cc diesels of their own. This gave rise to the 1/2 cc diesel "revolution" of 1951/52, which foreshadowed the similar contest of 1959 featuring 0.8 cc (.049 cu. in.) glow-plug motors. IMA's entry into the 1/2 cc arena was their Frog 50, of which more in its place below.

The attractiveness of these small engines to mainstream British modellers arose from a number of factors. They weren't significantly cheaper than many of their larger brethren, but in the far-off days when most aeromodellers lived up to their hobby's name by actually building their own models, airframes for these little units were both easy and less expensive to build. Moreover, such models, particularly those of the control-line variety, could be flown on smaller fields, and finally, the noise levels and fuel costs associated with running these little powerplants were far lower than those of the larger engines generally used by competition fliers.

Although the new breed of 1/2 cc diesels handily outperformed their sub-miniature 1940's predecessors and could actually give some of the contemporary American .049 glow-plug models a run for their money, they undeniably had their limitations. For one thing, they were far trickier to handle and more susceptible to damage than their larger counterparts, which made them less suitable for beginners. For another, their power output was still on the marginal side, especially when it came to the control-line field which was then so popular in Britain. Finally, they tended to wear out more rapidly than their larger contemporaries due to the greater sensitivity of smaller diesels to piston/cylinder wear in particular—the same amount of wear has a far greater negative effect with small bores.

For the above reasons, the 1/2 cc fad in Britain didn't last all that long, even though some of the protagonists remained in production for quite a few years. However, the previously-cited advantages of smaller engines remained in effect. Consequently, the continued growth in the popularity of control-line flying in Britain during the first half of the 1950's kept the pressure on domestic engine manufacturers to produce further updated designs having even more power while still remaining as light and compact as possible.

Once again, the lead was taken by Alan Allbon, who was now working in association with D-C Ltd Allbon wasted little time in getting to work developing a slightly larger model which gave a bit more urge while retaining much of the compactness of its 1/2 cc Dart companion. This appeared in October 1954 in the shape of the D-C manufactured 0.76 cc Allbon Merlin, which joined the long-established Mills .75 in the British 3/4 cc diesel category. With a measured output of 0.058 BHP @ 13,000 rpm as reported by Ron Warring in his very positive December 1954 Aeromodeller test, the Merlin was a far stronger performer than any of its 1/2 cc rivals as well as being very competitively priced at £2 7s 6d (£2.37), thus undercutting even its smaller Dart stablemate which continued to sell for £3 4s 2d (.£2.21).

D-C Ltd clearly saw the Merlin as an alternative to the Dart rather than as a competitor—the Dart remained in production alongside the Merlin, as it was to do for many years to come. However, the Merlin did have the distinction of being the first British second-generation rotary valve diesel design to appear in the 3/4 cc displacement which had hitherto been the preserve of the rather old-fashioned side-port Mills .75 and the American McCoy and OK .049 cu. in. diesel models. In hindsight, it's now also possible to see that it signaled the onset of a price war among British manufacturers which was eventually to stifle the development of new models and cut profit margins to the point of non-viability for all but a few of the more specialized producers.

IMA were relatively slow to respond to the challenge thrown down by the Merlin—the Frog 50's low selling price at the time of only £2 5s 0d (£2.25) was significantly lower than the Dart and lower even than the Merlin and made it the lowest-priced model engine then available to the British modeller. This price must have cut the profit margin to the bone, yet despite this the engine does not appear to have achieved the kind of sales figures that would be necessary to generate large overall profits. Taken together, these factors doubtless convinced IMA management that there was insufficient economic justification for further development work in the under-1 cc category.

However, the Merlin was undoubtedly a very useful performer which sold at a most attractive price and did a creditable job of flying small sport control-line and other models, as I know from extensive first-hand experience. It deservedly became extremely popular and certainly attracted its share of sales attention away from the little 1/2 cc designs, including the Frog 50. Perhaps more significantly, it also drew sales away from the 1 cc competition.

The success of the Merlin must eventually have made it clear to IMA both that there was a viable market for under-1 cc engines and that they could no longer ignore the need to develop a further upgraded model in the under-1 cc category if they wished to remain in competition for a share of that market. Since it is with the IMA involvement in this phase of the story with which we are concerned here, from this point on we will focus on their Frog range.

1952—Forerunner of the Frog 80—the Frog 50

IMA's entry into the 1/2 cc sweepstakes had taken the form of the neat and compact little Frog 50 diesel. With its FRV induction, internal flute bypass passages and radial porting, this was basically a scaled-down version of IMA's contemporary 1.5 cc model, the well-established and highly regarded Frog 150. Its bore and stroke of 8.71 mm and 8.38 mm respectively gave it a displacement of exactly 0.50 cc, as its name suggested. It was the smallest engine ever to appear under the Frog banner.

After spending much of 1951 in the development stage and then having to wait its turn in IMA's busy production program, the little Frog was finally announced in a full-page three-tone advertisement which appeared in the February 1952 issue of Model Aircraft. The introductory price of the engine was a highly competitive £2 9s 6d (£2.48), although this was reduced to £2 5s 0d (£2.25) in February 1953. Interestingly, the illustration of the engine in this advertisement showed a single-armed compression lever. Although this illustration was to appear in Frog advertisements for several years, I have yet to encounter an actual example having this feature. It seems that the illustration may have been based upon a prototype.

The Frog 50 was described in Peter Chinn's Accent on Power column in the April 1952 issue of Model Aircraft and was subsequently featured in published engine tests by (presumably) Chinn (Model Aircraft) and Ron Warring (Aeromodeller), both of which appeared in the respective magazines' August 1952 issues. The two testers found broadly comparable levels of performance—Chinn obtained 0.042 BHP @ 12,600 rpm while Warring achieved a somewhat lower figure of just over 0.030 BHP @ 12,700 rpm. These figures are consistent with other tests by the two individuals—Chinn tended to find more power than Warring for the same engine and commonly also found that power at higher peak rpm, although not in this instance. Presumably this was down to differences in the respective engine management and testing techniques.

Both reviews were generally favorable, although the fiddly little compression screw, needle alignment, starting qualities and general "finicky-ness" of the engine came in for some criticism from both reviewers. The manufacturers moved quite quickly to address the compression screw issue, replacing the original miniscule component with a more substantial item which was far easier to grip. This change appears to have occurred after perhaps some 3000 examples of the original Mk I were produced—my own example serial number 3513 has the larger comp screw while retaining the original Mk I cylinder. Kevin Richards also has a record of engine number 4321 of this type. That example also had a longer fuel tank fitted.

This relatively minor variant has sometimes been referred to as the Frog 50 Mk II, with the following model referred to as the Mk III. However, neither the factory nor contemporary reviewers ever applied such designations—both the first and second variants of the Frog 50 were designated as Mk I's.

In around February 1955 a further revised version was introduced which was referred to both by the factory and by contemporary reviewers as the Frog 50 Mk II. This variant addressed one of the major handling concerns expressed previously regarding the original model by raking back the needle to get the fingers further away from the prop. The updated model also featured revised transfer porting arrangements using external bypass passages feeding upwardly-inclined drilled transfer ports in place of the former internal flute bypass arrangements. The longer fuel tank which had been fitted to some of the later examples of the Mk I had now become standard, as had the enlarged comp screw. Finally, the crankcase was strengthened somewhat by the addition of an integrally-cast "strap" around the outside diameter in the vicinity of the con-rod’s operating location.

Sales of the Frog 50 were evidently just about sufficient to justify continued production for some 5 years, albeit at a relatively modest level of perhaps at most some 2500 units per year if known serial numbers are anything to go by—the highest serial number of our present acquaintance is Mk II engine number 11538 which is owned by Bob Allen. Kevin Richards recalls that the handling difficulties which were experienced by both testers and owners (himself included!) contributed to a progressively increasing degree of sales resistance—he recalls seeing one in his local hobby shop in 1959 (three years after production had ceased!) which was still there when the shop closed its doors in 1965!

A mini-test of the Frog 50 Mk II appeared rather belatedly in the February 1957 issue of Model Aircraft. Performance was essentially unchanged by the modifications, but the tester (almost certainly Peter Chinn) reported significantly improved handling.

However, by this time the Frog 50 had become irrelevant to IMA's evolving plans for their model engine range—the company was already in the process of replacing it with a new and somewhat larger model bearing little resemblance to its smaller predecessor. Indeed, the appearance of the new model had been announced in the previous issue! This being the case, the publication of a test on a discontinued model would seem at first glance to indicate that Chinn had been caught on the wrong foot! However, this is highly unlikely—Chinn was unusually well-informed regarding the plans of the various British model engine manufacturers and indeed was often consulted by them when developing new models.

In reality, a careful reading of the report clearly indicates that it had actually been written some 9 months previously and had presumably been submitted for the Editor to use as a "filler". The text refers to the Frog 50 Mk II having been "introduced just over a year ago " and that the Mk I version of the same engine had first appeared "three years earlier". Since the latter date is well-documented as February 1952, simple arithmetic dates the writing of this report pretty firmly to around April of 1956. The insertion of the report after the discontinuation of the engine and without correcting the redundant timeframes mentioned in the text was clearly an editorial oversight rather than a miscue by Chinn.

1957—The Frog 80 Mk I Diesel Appears

The market entry of the Frog 80 diesel was announced through the publication of a full-page two-tone advertisement in the January 1957 issue of Model Aircraft. The quoted selling price of the new engine was a very competitive £2 5s 0d (£2.25), matching that of the Frog 50. More engine for the same money!

The new engine was the subject of comment in the regular "Motor Mart" feature in the February 1957 issue of Aeromodeller—the same month as that in which Chinn's mini-test of the Frog 50 Mk II belatedly appeared in the rival Model Aircraft publication. Production of the Frog 50 appears to have ceased more or less concurrently with the 80's appearance—it certainly disappeared from IMA's advertising at this time, although examples seem to have remained available from dealers for some months thereafter.

In the "Motor Mart" article, Aeromodeller served notice of their intention to publish a Ron Warring test of the new model in their March 1957 issue. Chinn and Model Aircraft were quick to react, and tests appeared in the March 1957 issues of both magazines.

Warring was able to closely approach the figures that he had obtained a few years earlier for the rival Allbon Merlin—he extracted 0.051 BHP @ 11,000 rpm. While the actual peak power figure was some 12% down on that of the Merlin, it was released at significantly lower rpm, making the Frog 80 far more of a torque-producer than its Manx rival and thus able to swing a larger and hence more efficient airscrew. The handling too came in for considerable praise.

Peter Chinn's contemporary test in Model Aircraft was equally positive. He began by stating that the new 80 was "a better engine in every respect" than the 50 which it was intended to replace. He commented that "its starting, general behavior and running qualities are most certainly superior to those of the earlier model".

An output of 0.064 BHP was recorded at a speed of 11,800 rpm, both figures being somewhat higher than those obtained by Warring. As mentioned earlier, this is a typical characteristic of the tests conducted by the two individuals on the same engine—Chinn tended to find higher peak output figures at higher rpm than Warring.

Regardless, on the basis of the above tests as well as its low price it's clear that IMA had a worthy contender on their hands in the 3/4 cc sports diesel category. Let's have a look at this engine to see what made it tick.

The Frog 80 Mk I—Description

Given the fact that our initial motivation for commencing this review was to describe IMA's entry into the 1959 British 1/2 A glow-plug sweepstakes with their Frog .049, it may seem redundant to dwell at length upon the diesel predecessor of that model. However, this is far from the case given the fact that the Frog .049 was in effect simply a glow-plug conversion of the earlier diesel. Hence an understanding of one informs our understanding of the other.

Beneath the skin, the Frog 80 Mk I was basically a more-or-less conventional reverse-flow scavenged crankshaft front rotary valve (FRV) model diesel of its era. However, it did possess a number of features which set it apart from its competition.

Bore and stroke were a near-square 10.16 mm and 9.91 mm respectively for a displacement of 0.804 cc (almost exactly 0.049 cu. in.). The engine weighed in at 54 gm (1.90 ounces)—a little more than the competing 49 gm (1.75 ounce) Allbon Merlin, and it was also considerably more bulky than its rival. It did however undercut the Merlin on price, and this was achieved with no evidence of undue corner-cutting in either design or production terms.

Looking at one of these engines today, its most striking visual attributes are the twin exhaust stacks and the deeply finned cylinder head. But the real heart of this design is clearly the cylinder assembly. The engine uses a case-hardened steel cylinder liner having integrally-machined cooling fins. There are four thin fins above the exhaust stacks, with a thicker flange at the top immediately below the cylinder head to add some stiffness at that point.

The cylinder is topped by a pressure die-cast aluminium alloy cylinder head having quite deep cast-on fins. The cylinder and head are jointly secured to the pressure die-cast alloy crankcase by a pair of long 8 BA screws which pass through holes drilled fore and aft through both the cylinder head and the cylinder fins to engage with tapped holes formed in suitable expansions formed in the pressure die-cast main crankcase. The cylinder is axially located in the crankcase casting by the exhaust port flange fitting into an annular recess provided in the upper crankcase for this purpose. A gasket is used beneath the exhaust port flange to ensure a seal.

The main problem which can arise with this assembly is distortion due to the uneven distribution of the hold-down forces around the cylinder—an inevitable consequence of the use of only two screws. This is a particular issue when dismantling a Frog 80 for any reason—it can be difficult to re-establish precisely the same "set" in the cylinder upon re-assembly. This is one engine that is definitely best left alone once run in with the cylinder assembly fully tightened. It's also important to ensure that the hold-down screws don't work loose.

The steel contra-piston is highly unusual for a British engine in using a synthetic rubber O-ring to maintain a gas seal. It also has a flange at the top which prevents it from being screwed down too far into the bore and risking either pre-ignition or a hydraulic lock—a good "beginner" feature. The design in fact mirrors that used in the earlier OK Cub diesels which had appeared in the USA beginning in late 1953.

This system works perfectly in practice, although the rather loose "feel" of the O-ring contra piston takes some getting used to. Chinn upheld the system quite strongly in his test report, stating that it functioned extremely well in operation. He recalled having defended the use of O-rings in the earlier McCoy and OK diesels, and saw no reason to change his views. He also mentioned that the O-rings in question were manufactured by the Burtonwood Engineering Co, who had reportedly estimated the working life of the O-rings at something in excess of 100 operating hours.

The absence of any real friction between the contra piston and the cylinder means that compression run-back would be a major problem in the absence of any means to control it. To deal with this issue, the Frog 80 features a nylon insert in the cylinder head which grips the comp screw sufficiently tightly to maintain the compression setting under all normal operating conditions. This idea too was borrowed from the McCoy and OK Cub diesels, although the material used was different. It is very effective in practise, and the idea was also applied to the later Frog 349 models, albeit without the O-Ring seal on the contra-piston.

Turning now to the porting, the cylinder is provided with twin exhaust slots sawn through the substantial exhaust port flange. These two ports feed directly into the two exhaust stacks, one on each side. Twin transfer slots are provided fore and aft immediately below the exhaust flange at 90 degrees to the exhaust ports. The two apparent bypass passages cast beneath the exhaust stacks at the sides are in fact dummies—the bypass is actually formed by the space between the inner crankcase wall and the lower cylinder outside wall. This space is interrupted at two points fore and aft by the expansions for the hold-down screws. Somewhat awkwardly, these interruptions occur directly in front of the two transfer slots, which are partially obstructed and are in effect fed from both sides as a result.

The flat-topped cast iron piston is of relatively lightweight construction. The gudgeon pin is 0.125 in. diameter and links the piston to a forged alloy connecting rod. This in turn connects to a one-piece steel crankshaft through a 0.141 in. diameter crankpin. The crankweb is a plain unbalanced disc and the crankshaft journal diameter is a nominal 0.250 in. with a central gas passage of 0.140 in. diameter and a round crankshaft induction port. It appears that IMA may have experienced problems with shaft breakages with the earlier examples, since the diameter of the central gas passage was later reduced to 0.125 in., presumably to increase the strength of the shaft.

The shaft runs in a plain bearing formed directly in the material of the crankcase casting. The aluminium alloy prop driver is secured by being force-fitted over a splined protrusion at the front of the shaft behind the prop mounting thread. A conventional prop nut and washer are used to retain the airscrew.

The engine uses a standard Frog 150 needle valve assembly which is angled back and slightly upwards to the right (facing forward). This keeps the fingers well clear of the prop disc but makes sidewinder mounting in a control-line model a bit problematic since the needle cannot be re-fitted from the opposite side. A spring clip engages with a serrated brass thimble to maintain needle settings very effectively.

The backplate is another pressure die-casting in light alloy which is held in place by a pair of 8 BA screws and is fitted with a gasket to ensure a seal No tank was fitted as supplied, but an accessory back tank was offered from the outset as an optional accessory at a cost of 3s 6d (18p). This was completely fuel-proof, being made of nylon with a metal front face. It was secured by the same two screws that held the backplate in place. This tank remained available for the duration of IMA's manufacture of the Frog 0.8 cc models, but none of the various diesel models were ever at any time supplied with tanks out of the box—they remained optional accessories throughout.

The price war between British manufacturers which we mentioned earlier was now well and truly underway, and the new Frog engine was clearly designed with a view towards minimizing production costs. For instance, the only machining done on the crankcase casting was the reaming and honing of the main bearing to fit, the drilling of the mounting holes and the drilling and tapping of the four holes for the various assembly screws.

The result was a well-made, easy starting and individualistic-looking engine having a useful "sports" performance for its displacement and selling for a most reasonable price. Ron Warring characterized it as "a real winner for popular use". And indeed it did find its fair share of buyers over the four years following its introduction, competing with the Allbon Merlin and the Mills .75 for sales in the 3/4 cc diesel category. My first diesel was in fact a somewhat "experienced" and extremely second-hand example of one of these engines, and I got a lot of good use out of it.

All of the engines carried hand-engraved serial numbers, in keeping with other Frog designs. No attempt was made by IMA to distinguish between different models through the use of model identification letters in the serial number—the engines of each type simply started at one and went on up from there. This of course created duplicate serial number sequences among the various models produced by IMA. The highest Frog 80 Mk I serial number of my present acquaintance is 14021, strongly suggesting that at least that many were made. But overall production could have been substantially higher—our present sample is far too small to provide conclusive information in that regard.

It's worth noting that the later examples of the Frog 80 Mk I are very slightly different from the earlier ones—my own engine number 9710 (which is NIB) exemplifies this minor variant. The cylinders and comp screws of these examples were blued rather than being left in their natural steel finish, and the cylinder head die was amended to create a slightly taller component with an emergent hemispherical top interrupting the fins, together with a longer comp screw spigot. In addition, these engines featured the previously-mentioned smaller 0.125 in. diameter central gas passage in the crankshaft. But these changes are insufficient to justify calling this a different model—it's merely a point of which to be aware.

1959—the British Glow-Plug Revolution

Now we get to the meat of the matter! We noted earlier that throughout the 1950's British modellers had generally remained true to the diesel for all but a few specialized modelling applications. However, overseas progress in the design of glow-plug engines for purposes other than racing was accelerating as the fifties drew on, and by the time 1959 rolled around the stage was set for a major shift in the perception of the glow-plug motor in the eyes of British modellers.

Things got rolling with the early 1959 introduction of the excellent Merco 29 and 35 glow-plug stunt engines, which joined the well-established and highly-regarded Frog 500 in this category. This event was widely publicised and showed British modellers that domestic manufacturers were fully capable of producing world-class engines of that type. The innovative and technically very successful Davies-Charlton Tornado Twin of 5 cc displacement was another outstanding British glow-plug achievement in the same year, and improved versions of the renowned ETA racing engines also continued to appear.

Moreover, the importation of the excellent glow-plug motors being produced in Japan by O.S., Enya and Fuji (among others) was on the rise, spurred on by some very positive published test reports and high-profile contest successes. In addition, the growing popularity of radio control was making British modellers increasingly aware of the superior throttling characteristics of glow-plug engines. Taken together, these initiatives resulted in the British marketplace becoming increasingly receptive to the broader use of glow-plug ignition in place of the hitherto almost universal diesel.

In the smaller displacement categories, the importation to Britain of the Cox range of 1/2 A (.049 cu. in.) glow-plug motors had begun in the late 1950's through the AA Hales organization, and people had certainly taken note of the excellent quality and performance of these little units. The major objection in Britain to the use of the Cox 1/2 A models (prior to the 1959 Space Hopper) was their adherence to radial mounting—British modellers had by then come down firmly in favour of beam mounting as the preferred standard. In addition, the fact that one was more or less forced to use the relatively bulky built-in tank was widely seen as a disadvantage—British modellers generally preferred to choose their own tank arrangements depending on the application. I can clearly recall flying field criticism of the Cox engines to this effect.

In early 1959 a number of British manufacturers evaluated the situation and decided pretty much simultaneously to enter the 1/2 A glow motor market themselves. This led immediately to a virtual replay of the 1951-52 half-cc diesel revolution, albeit this time with 1/2 A glow-plug motors as the contestants. One of the participants in this chain of events was of course International Model Aircraft.

Now there were three potential pathways to entry into this market. The first was to develop the company's own design, drawing as always upon the better attributes of established and well-proven designs already on the market but incorporating a blend of such features intended to enhance the engine's attractiveness to British buyers. Although development costs might be expected to be higher, this should by rights result in a better overall design. However, the only company to take this route was Keilkraft with their Cox-influenced but nonetheless original Cobra .049 which was made for them under contract by the firm of John Rodwell Ltd of Hornchurch in Essex. In many respects this was the best of the British 1/2 A glow-plug contenders, although it did not meet with the success that it deserved and faded from the scene quite quickly. See our companion article on the Cobra for more information.

Another option was to side-step the entire development process at one fell swoop by simply purchasing the rights to an established design which had already been fully developed and proven by others. This was the route taken by Allen-Mercury. The design with which they became associated was that of the then-current Wen-Mac .049 from America. Their A-M .049 was to all intents and purposes a straight Wen-Mac clone, of which more elsewhere.

The third possibility was to take the very simple route of producing a glow-plug version of an established diesel design already on the company's books. This was a time-honoured approach on the part of British manufacturers, having been adopted at various times by such makers as ED, Davies-Charlton, Allbon, AMCO and JB, to name just a few. Indeed, IMA had themselves gone this route a few years previously with their 149 Vibramatic model.

Such an approach had the benefit of side-stepping the bulk of the development costs very nicely and would maximize the use of components and tooling which were already in the company's production program, thus minimizing production costs. However, experience had clearly demonstrated that the simple switch from diesel to glow-plug ignition rarely results in the best-possible performer—none of the earlier glow-plug conversions of diesel models had matched their diesel progenitors in performance terms. Nonetheless, this was the route taken by D-C Ltd and IMA with their Bantam and Frog .049 models respectively.

At long last, we're ready to take a look at IMA's entry into the 1959 British 1/2 A glow-plug sweepstakes—the Frog .049

1959—The Frog .049

The Frog .049 achieved what may well have been one of the objectives set by the IMA Board of Directors—it was the first of the British .049 glow motors to put in an appearance on the domestic market. The engine was announced in July 1959, thus beating the rival D-C Bantam and A-M .049 into the marketplace by at least three months and the Cobra by over a year. The initial selling price was set at £2 9s 6d, slightly higher than the 80 diesel which continued in production.

For better or worse, IMA had decided that the line of least resistance towards the development of their own .049 cu. in. glow-plug model lay in simply converting their existing and well-proven 80 diesel model to glow-plug ignition. However, it is to the credit of IMA's chief engine designer George Fletcher that he seems to have realized that a direct conversion might not yield the best results.

Accordingly, while IMA management seems to have directed Fletcher to control costs by maximising the use of components of the 80 diesel which were already in IMA's production program, Fletcher was given sufficient leeway to permit experimenting with changes in the cylinder arrangements to maximise the engine's potential in glow-plug form.

As one would expect under these circumstances, the resulting design necessarily remained very similar to the 80 diesel described previously, and the bulk of our earlier description of that model applies equally to the .049 glow unit. The only major differences were to be found in the cylinder assembly.

Obviously, the first things to go were the contra-piston and diesel cylinder head. A revised un-finned head was produced from turned aluminium alloy. This alone seems to indicate that the manufacturers anticipated relatively low production figures for the Frog 80—a casting produced by a modified diesel head die would have made more sense if large-volume production was anticipated.

The head was deeply spigoted into the upper cylinder, thus largely filling the space vacated by the missing contra-piston. A gasket was used to create a seal. The combustion chamber was of hemispherical form and the head was centrally drilled and tapped for a short-reach glow plug. It was secured by the same two-screw arrangement which had been used in the 80 diesel, with no supplementary screws to further seal the head to the cylinder.

This again is an assembly that is definitely best left undisturbed once an engine is run in and fully settled down—apart from the distortion issue mentioned earlier, re-establishment of a good head seal can be problematic. In addition, it's critical that the hold-down screws not be allowed to work loose—if they do, a blown head gasket is inevitable.

The steel cylinder itself was also changed quite significantly. The twin opposed exhaust ports were considerably larger than those used on the Frog 80 diesel, being both wider and taller. In addition, the length of the cylinder below the exhaust port flange was slightly shortened. The piston used in the .049 had a slightly higher crown and a consequently longer skirt than that of the 80, and the net result was a slightly later opening of the exhaust ports in the .049 compared with the 80.

This later exhaust opening was made possible both by the larger port area and by the fact that the transfer arrangements were also substantially altered. In place of the two transfer slots milled fore and aft into the lower cylinder of the 80 diesel there were now four upwardly-angled drilled holes arranged fore and aft in pairs between the two exhaust slots. As with the Frog 80, these were fed from the space between the inner wall of the upper crankcase and the outer wall of the lower cylinder, but now there was one transfer hole on each side of the two protrusions which accommodated the cylinder hold-down screws. The presence of those two protrusions thus became irrelevant in terms of gas flow.

More importantly, the revised transfer port design allowed for a degree of overlap between exhaust and transfer, something which had not been possible with the original design. Hence the slightly later exhaust opening could be accomplished with no loss in transfer duration—in fact, the .049 had a slightly greater transfer period than the 80 despite the taller piston crown.

Another change was the cylinder finning. The cooling fns were integrally machined as before, but there was now one less fin between the exhaust stacks and the upper cylinder flange, the cylinder itself being slightly shorter and the fins being more widely spaced. The cylinder walls were also fractionally thinner. Finally, the cylinder was blued instead of being left in its plain case-hardened state as before. It seems likely that it was at this point that the companion Frog 80 Mk I received its own blued cylinder and taller head, as mentioned earlier. The use in both models of the revised shaft with the slightly smaller central gas passage seems also to date from the introduction of the Frog .049.

The only other change was the replacement of the standard prop-nut and washer of the 80 with a rather bulky turned and knurled aluminium alloy spinner nut. This was provided to allow owners the option of using a cord to pull-start the engine, assisted starting being something of a preoccupation with British model engine manufacturers at the time. However, the engine was such an easy hand-starter that measures of this sort were quite unnecessary, and all the spinner did was to add some extra bulk and weight to the engine.

In all other respects, the Frog .049 was identical to its 80 diesel stablemate. Bore and stroke were unchanged at 10.16 mm and 9.91 mm respectively for a displacement of 0.804 cc (0.049 cu. in.). As one would expect, the glow-plug version was very slightly lighter than its diesel counterpart at 51 gm (1.8 ounces). It would have been a little lighter yet if it hadn't been for that overly bulky spinner nut...

As it was, the Frog .049 was the heaviest and by far the most bulky of the various British 1/2 A glow-plug contenders. It was also the only one which could be directly compared with the diesel model upon which it was based given the fact that both versions had identical displacements (unlike the D-C Dart and Bantam) and shared many components. Let's see how it fared in that respect.

The Frog .049 On Test

Oddly enough, there was a considerable delay between the appearance of the Frog .049 and the publication of its first test. Initial tests of new British models usually followed hard on the heels of the engines' introduction, but not so in this case. Peter Chinn and Model Aircraft were first into print, with Chinn's test of the Frog .049 appearing in the December 1959 issue, a month in advance of the publication of Ron Warring's test in the rival Aeromodeller magazine.

Chinn's report was interesting insofar as he found that starting, while very dependable, was not quite as quick as might be expected. He reported that after priming, some 30 seconds of flicking were typically required to achieve a start, and even cord starting required up to five pulls. I can only comment that this is completely contrary to my own experience—on the basis of three examples of my direct hands-on acquaintance, I've found that the Frog .049 is one of the easiest starting small engines of them all, with only a couple of leisurely flicks being required following a prime and a hand-turn to get a "bump" in the usual glow-plug fashion. The thought occurs that perhaps Chinn had a plug problem?

Chinn accurately described the design changes between the 80 and the .049 and reported even and consistent running, albeit with a slightly higher level of vibration than might be expected. In hindsight, it does appear that this motor with its slightly heavier piston might have benefited from a degree of counterbalance being applied to the crankweb.

The reported peak power figure was 0.042 BHP at around 12,700 rpm. This output is some 33% down on that obtained by Chinn in 1957 for the 80 Mk I diesel, albeit at slightly higher revs. It is however significantly higher than the figure obtained by Chinn a few months later for the rival D-C Bantam. Chinn could only extract 0.035 BHP at 12,600 rpm from that rather anaemic model.

Chinn refrained from giving his usual closing summary of the engine's qualities—he confined himself in this instance to simply reporting the facts recorded during his examination and subsequent test. Reading between the lines, one gets the impression that he was somewhat underwhelmed by the Frog .049!

Aeromodeller magazine may have been beaten to the punch on their test of the Frog .049, but they more than made up for this in the following month by publishing an unusual triple test in their January 1960 issue. This was a head-to-head encounter between the three British glow-plug .049 contenders which had successively appeared in the latter half of 1959—the Frog .049, A-M .049 and D-C Bantam. Warring's tests of the D-C and A-M offerings scooped Chinn's by several months.

However, it would appear that in their haste to get these tests into print, Aeromodeller fell into a trap when it came to the D-C Bantam—all the evidence points to the near-certainty that they knowingly or otherwise tested the prototype version of the Bantam, which had a far higher performance than the production model. This led to greatly inflated expectations of the Bantam among contemporary modellers, which were brought crashing to the ground both by actual experience with the engine and by Chinn's far more representative test report which appeared in the March 1960 issue of Model Aircraft. The credibility of both Warring and D-C Ltd was undoubtedly harmed by this incident. See our separate article on the Bantam for the gory details.

Returning to the Frog .049, Warring's test was generally very positive—far more so in fact than Chinn's had been. The handling of the engine came in for the highest praise, in stark contrast to Chinn's evaluation but very much in line with my own experience of several examples. The running qualities also came in for positive comment, a view which I can also endorse from personal experience. However, Warring (like Chinn) was unable to approach the performance figures which he had obtained almost 3 years previously for the Frog 80 Mk I diesel. Instead of the figure of 0.051 BHP @ 11,000 rpm for the Frog 80, Warring could only extract 0.037 BHP from the Frog .049 on 15% nitromethane, albeit at a slightly higher speed of 12,000 rpm.

Once again, we see the typical pattern of tests on the same engine conducted by the two individuals—Chinn recorded both a higher peak output and a higher peaking speed than Warring. It is however interesting to note that the proportional shortfall in peak power recorded by both individuals for the .049 compared to the 80 diesel is very similar—Warring recorded a 28% shortfall as compared with Chinn's 33%. In that respect, the two tests are consistent.

In summary, IMA's decision to join the ranks of the British .049 glow-plug manufacturers by simply converting their existing diesel model had seemingly backfired. The direct consequence of their taking this approach was that they ended up with the heaviest, most expensive and most bulky of the various contenders, and moreover one which didn't approach the performance of its diesel counterpart. Hardly a recipe for success!

The Frog .049 in the Marketplace—A Personal Recollection

At the time when the various British .049 glow-plug models appeared, I was just getting started on my power modelling career. My parents had given me a brand new Series 2 ED Bee for my 12th birthday in 1959, and I also had ratty but still highly serviceable examples of the Allbon Merlin and the Frog 80 Mk I, both obtained dirt cheap in "experienced" condition.

I had already developed a sense of curiosity regarding glow-plug engines, so I read Warring's January 1960 triple test in Aeromodeller with great interest. I have to confess that Warring's report left me feeling quite indifferent to the Frog .049—it was by far the bulkiest, heaviest and most costly of the three contenders as well as being the least sturdy performer of the three, based on Warring's figures. In addition, I already had a Frog 80 diesel, which clearly outperformed the .049 glow version by a considerable margin. Why switch?!?

I was in fact one of those who was lured into a false sense of admiration for the D-C Bantam, misled by Warring's extremely inflated test results for that model. By the time I realized my error following the publication of Chinn's far more representative test report as well as my own sorry experiences with the totally gutless second-hand Bantam that I quickly picked up on the cheap, the damage was done—I was off the Brit 1/2 A glows and back to using my trusty little diesels. I would have given the far more progressive Cobra .049 a try, but it never showed up in any of the hobby shops in my area.

My reaction was evidently typical—the Frog .049 appears to have achieved very limited sales success. All indications from known serial numbers are that perhaps only some 2000 examples were produced in total. It's likely that production only continued for as long as it did because most of the components were in production anyway for the far more successful 80 diesel and the incremental cost of throwing the odd batch of .049 cylinders and heads into the mix was pretty low.

IMA did make quite serious efforts to promote the engine. They continued to feature it front and centre in their advertising throughout much of 1960, offering it in two forms—the basic no-frills model at £2 9s 6d (£2.47) and the so-called "Presentation Set" version at £2 17s 6d (£2.87). The latter offering included the accessory back tank mentioned earlier as well as a tommy bar for the spinner nut and a matching Frog airscrew. However, the engine itself was unchanged. This was the sole offering of any of the Frog 0.8 cc models which included the tank. IMA also developed flying model kits specifically intended for the Frog .049.

However, IMA were facing a number of stark realities about which little or nothing could be done. Firstly, there was no hiding from the fact that the Frog .049 was by far the most expensive of the various British 1/2 A glow-plug models. The late 1959 appearance of the D-C Bantam at only £1 14s 6d must have come as a terrific shock to IMA management—they had clearly had no idea that they would be facing price competition at that level. The A-M .049 at £1 19s 6d did nothing to help, and the Cobra .049 at £1 19s 6d offered further lower-cost competition despite its later appearance on the market.

IMA management must have been painfully aware of this situation. The fact that they never rolled the price of the Frog .049 back to match seems to indicate that they had no leeway to do so if they were to maintain a reasonable profit margin. Evidently they had only two alternatives—either sell the engine at the established price or concede defeat and withdraw it.

The first of these courses of action was hampered by the fact that they had the heaviest, bulkiest and most expensive of the various competing models. Not only that, but it offered no performance advantage over most of the competing models. There was thus no basis for any potential customer to justify the purchase of the Frog .049 over any of its competitors, and there was nothing that IMA could do about this short of undertaking a complete re-design from scratch. This would clearly have been uneconomic given the competition which they now faced.

This is a good example of an often overlooked outcome of the price war which had by now been raging for some time between British manufacturers. British modellers at the time were more than happy to take advantage of this situation, but failed to see the long-term negative effects arising from the consequent reduction of profit margins to a bare minimum. In the context of the relatively small British market, there was now little economic justification for the development of new models—the necessary financial incentives simply weren't there. As a result, British manufacturers had entered a period of standing pat on their existing designs or minor variants thereof, and this led to complete stagnation in the face of developments elsewhere. It's a little ironic that both IMA and D-C Ltd, who had done much to provoke this price war, were both eventually engulfed by its effects.

Apart from the commercial aspect, the real crusher was the inescapable fact that the Frog .049 was after all simply a glow-plug version of an established diesel. That model was considerably more powerful than its .049 glow relative and moreover cost less. So why would anyone contemplate a switch?!?

Looking at things in light of the above comments, it becomes crystal clear in hindsight that the Frog .049 never really had a chance once the various competing models arrived on the scene and although it appears that they continued to sell the engine in limited quantities for some time after, by early 1961 the engine had disappeared from IMA's advertising.

1961—The Frog 80 Mk II

The changes to the cylinder of the Frog 80 to produced the Frog .049 left IMA in a situation in which they were producing two very similar engines which differed mainly in their cylinder and cylinder head arrangements. There would be clear cost benefits if the commonality of parts could be further increased. In particular, utilizing a common cylinder would represent a real saving.

In late 1960, IMA finally decided to do something about this. The result was the early 1961 appearance of the Frog 80 Mk II diesel. The main change from its Mk I predecessor was the adoption of the .049 cylinder with its larger exhaust ports and drilled transfer ports, in place of the smaller exhausts and milled transfer slots used in the original 80 cylinder. This resulted in the only externally-visible difference between this version and its predecessor—there was one less cooling fin to go along with the slightly taller head which was carried over from the later examples of the Frog 80 Mk I.

Despite this change, the piston of the original 80 was retained unaltered. It will be recalled that this component had a lower crown than the taller piston used in the .049, and the result was earlier opening of both the exhaust and transfer ports by comparison with both the original 80 Mk I and the .049.

Another change was the dropping of the O-Ring contra piston in favor of a conventional lapped component—although this must have represented an increase in production cost, customer feedback had evidently shown that die-hard diesel users preferred the "feel" of a lapped contra. Apart from these amendments, the Frog 80 Mk II was unchanged from its predecessor.

Despite the increased cost of the lapped contra piston, the net economies arising from the use of a common cylinder permitted a modest price reduction, from £2 5s 0d (£2.25) to £2 2s 9d (£2.14). This still left the Frog 80 Mk II at a price disadvantage when compared with the D-C Bantam, for example, but there's no doubt at all that it handily outperformed the Bantam.

This variant was the subject of a further Ron Warring test report which appeared in the November 1961 issue of Aeromodeller. Once again, Warring was generally very complimentary regarding the engine's handling and running qualities, although he commented on the increased noise level resulting from the earlier exhaust port opening! He was able to measure a modest but nonetheless useful performance improvement as well, finding a peak output of 0.057 BHP @ 11,000 rpm—roughly a 10% improvement in output. The peaking speed was unchanged from the figure obtained by Warring for the original Frog 80, but there had been an increase in torque, leading to the higher peak BHP figure. Another welcome attribute was the far flatter power curve, which resulted in 0.05 BHP (the peak output of the former model) being exceeded at all speeds between 8,000 and 14,000 rpm—an unusually wide useable speed range.

In this instance, Aeromodeller were unchallenged on the testing front—the rival Model Aircraft magazine never ran a test on the Frog 80 Mk II. Indeed, the November 1961 Aeromodeller test was destined to be the final appearance of a Frog 0.8 cc model in a published test report.

1962/64—A Change of Manufacturer

The Frog 80 Mk II diesel continued to be produced by IMA into the first half of the year 1962. At that point, however, things changed completely when a decision was taken by Lines Brothers, the parent company of IMA, to end production of the entire Frog line-up of flying model kits and engines. The reasons for this decision are now obscure, but presumably the flying model side of IMA's business simply wasn't seen as sufficiently profitable to justify its continuation—no doubt the aforementioned price war contributed to this conclusion. The popular Frog plastic kits did however continue in large-scale production by IMA for some years to come. Interestingly enough, so did the production by IMA of Pedigree dolls (!), a line which IMA had been producing since 1950 alongside the plastic kits! It's all plastic molding, after all.

At the time when IMA's production of the Frog model engine range ceased, there were significant stocks of completed engines and parts in existence, and these continued to be sold off by the AA Hales organization of Potters Bar in Hertfordshire, just north of London. Hales were one of IMA's major distributors and also produced their own range of model kits under the "Yeoman" label—anyone remember the "Dixielander", the "Bantam Cock", and the "Scorcher"? They later became a full member of the Lines Group of companies.

The cessation of engine production by IMA left George Fletcher looking for fresh employment, which he soon found with the nearby ED company at West Molesey. Interestingly enough, he replaced his former IMA colleague Gordon Cornell as chief designer for ED, the latter having joined ED in 1959 and left in 1961 following some philosophical differences of opinion with the then-current ED management.

As a result of their ongoing marketing of the existing stocks of Frog engines, the Hales organization clearly detected a level of continuing demand for the Frog range which would not be met solely through the liquidation of existing stocks. In 1964 they became a full member of the Lines Group, at which point arrangements were made with Lines Brothers to have all of the Frog engine designs and tooling transferred to IMA's old rivals D-C Ltd on the Isle of Man. Thenceforth the Frog engines were manufactured by D-C Ltd under contract to AA Hales. The Frog 80 Mk II diesel was one of the models initially manufactured by D-C Ltd under this arrangement.

Not all of the Frog engine range went into reproduction. The abandoned designs included the outstanding 1.5 cc Viper diesel and its glow relative the Venom, as well as the Vibramatic 149 diesel and glow models. Another casualty was the central subject of this article, the rather unsuccessful Frog .049 glow model. However, it appears from surviving examples that a few examples of the Frog .049 and the 149 Vibramatic diesel were assembled by D-C from existing parts, presumably to use up a spare parts inventory for which there was by then little demand.

It must be said that the quality of the Frog engines appeared to suffer somewhat by the change of manufacturer. It's my personal impression that the 80's (and other Frog models) made by D-C Ltd never quite matched the IMA originals for consistency in terms of quality. However, they generally worked OK and at least spare parts continued to be available for a few more years.

Examples of the Frog engines manufactured by D-C Ltd are easily identified by the fact that they carry a model identification letter in front of the serial number. It appears that when AA Hales arranged for D-C Ltd to resume production, they re-started the serial number sequence for each model at 1, simply adding a letter prior to the number to define the particular model. As far as we can determine, the identification letters were as follows:

A.049 glow †
B149 "Vibramatic" †
C80 Mk II
E80 Mk III
T249 BB
Z349 BB

† Probably assembled from existing parts by D-C Ltd rather than manufactured by them. The use of this system was a very sensible idea as IMA had simply numbered the engines of each displacement sequentially starting at one and had consequently generated complete sets of duplicate serial numbers! The approach taken by AA Hales eliminated the duplicates and made each number unique.

It's actually possible that the examples of the Frog 80 Mk II produced by D-C Ltd were also assembled from existing parts rather than being manufactured by D-C Ltd The Mk II engines produced by D-C Ltd may be distinguished from the IMA-manufactured examples by the C prefix attached to their serial numbers. The only such Mk II serial number which we have at present is C8860, which was NIB with all papers and thus had excellent provenance. As will be seen shortly, we know that D-C Ltd didn't manufacture the Frog 80 Mk II for very long, and the magnitude of this number implies that D-C Ltd did not restart the serial number sequence in this case, probably because they were already contemplating the replacement of the Mk II with a further updated model.

If this view is correct, then the possibility exists that a combined total of perhaps as many as 9,000 examples of the Mk II may have been produced by IMA and D-C Ltd during the three-year period between early 1961 and early to mid 1964. We'd need a good few additional serial numbers to be any more definite than that.

1964—The Frog 80 Mk III

Regardless of the true facts surrounding D-C Ltd's production of the Frog 80 Mk II, we know for certain that soon after they took over the manufacture of the Frog range in 1964, D-C Ltd began to manufacture a further minor variant of the engine which was referred to as the Frog 80 Mk III. The fact that this was seen by AA Hales as a distinct variant is demonstrated by the fact that, as reflected in the above model identification table, they assigned a different model identification letter to it. They also appear to have restarted the serial number sequence—we know of several Mk III engines having lower numbers than the highest Mk II number of which we are aware.

The engine was in reality little changed from the earlier Frog 80 Mk II. It featured the same porting and structural arrangements, with most of the components remaining interchangeable between the two models. However, both the exhaust and transfer ports were further enlarged. In addition, the hardened steel cylinder was stiffened somewhat through a thickening of the integrally-machined cooling fins which continued to be used, the number of these fins being unchanged. The top edge of the upper cylinder flange was slightly beveled as well, and the cylinder overall was some 0.025 in. longer than that of the Mk II.

Internally, the main change was the use of a slightly shorter con-rod in the Mk III, the gudgeon pin hole being relocated further down the piston skirt to allow for this while retaining essentially the same port timing. The reasoning behind this change is a bit obscure, since its effect would be to increase the side-thrust of the piston against the cylinder wall during operation—not at first sight a desirable outcome. It seems not unlikely that the change was motivated simply by a desire to further increase the commonality of parts in the D-C production program by using a standard rod that was already in the program—perhaps that of the Merlin or the Bantam. Up to the present, I have had no opportunity to check out this theory through direct comparison of components.

Another change which I for one did not view positively was the addition of D-C's "Quickstart" spring starting system to the Mk III. I always thought this to be one of the sillier and more unnecessary accessories ever produced by the British model engine industry—the engines to which it was applied were such easy hand starters that it was quite unnecessary, merely adding bulk, weight and cost to the product for no real gain. But there it was, and the fact that most owners agreed with me is reflected by the fact that almost all used examples of the Frog 80 Mk III that one encounters today, even the clean little-used ones, are missing the Quickstart clobber. Good riddance!

The only other apparent change was the use of a standard D-C needle valve assembly instead of the former IMA item with serrated brass thimble. The overall result was a really nice little diesel with a further-enhanced performance courtesy of the slightly enlarged ports. Thanks presumably to the inclusion of the starter spring set-up, the selling price had crept back up a little to £2 4s 6d (£2.22), but that was still a reasonably competitive price allowing for inflation.

Although it was never tested, there's little doubt that the Mk III Frog 80 diesel was the best version of that engine ever to appear—both Kevin Richards and I agree on that point on the basis of direct experience with multiple examples. They are really good runners!

End of the Road

As the 1960's drew on, it became increasingly obvious to most modellers that a number of the old-established British model engine ranges such as ED, D-C and Frog had stagnated to the point where they had become increasingly out of step with the emerging modelling marketplace. New models were eagerly awaited from established British manufacturers, but they failed to materialize for the most part, almost certainly for the economic reasons discussed earlier. Moreover, R/C flying was now predominant in place of the formerly popular control-line, and this in turn meant that glow-plug motors with their far superior throttling characteristics were very much in the ascendant.

It was inevitable that any model engine range which was based on "traditional" sports diesels would encounter increasing difficulty remaining afloat in such a marketplace. And indeed this was the case with the Frog range. As far as we can determine, AA Hales' final consumer market advertisement for the D-C-produced Frog engines appeared in the April 1966 issue of Aeromodeller magazine. And although IMA had continued to produce the Frog plastic kit range in great volume after 1962, they too ceased advertising in Aeromodeller after June 1966. Despite this, it appears that plastic kit production continued at some level up to 1971, when the parent Triang company entered receivership and the Frog moulds were sold to producers in the Eastern Bloc and elsewhere.

Following the collapse of the former Lines Brothers empire, the Tri-ang Works remained dormant on the site at Morden Road until 1973, when the facility was converted to serve as a production centre for a major bed manufacturer, Sleepeezee Ltd The location was re-numbered 61 Morden Road at this time. Sleepeezee remained at this location until 2008, when the site was cleared and re-landscaped to house the vast Big Yellow Box storage company, which rents storage rooms for the use of business and residential clients. This company continues to occupy the site today. There are no surviving traces of the former Lines Brothers factory, although much of the surrounding area remains in use for industrial purposes. My thanks to Sarah Gould of the Borough of Merton for this information.

The April '66 ad from AA Hales included the Frog 80 Mk III, so it seems to have survived to the very end of production of the mainstream Frog engine range, having remained in production in various forms for at least nine years—not a bad record! It's presently unclear how long, if at all, production of the Frog engines by D-C Ltd continued past this point in time. Kevin Richards has confirmed that despite the absence of any further advertising, new engines remained available through dealers until at least 1970. Kevin bought brand-new boxed examples of both the 349 BB and the 249 BB from his local hobby shop in that year, and he still has the 249 BB new in the box complete with 1970 date-stamped factory guarantee!

This cannot of course be taken as hard evidence of continued manufacture as of that date-it's entirely possible and even likely that Hales and D-C Ltd were simply selling off new-old-stock by that time. But it does confirm that the Frog engines remained available from dealers for a considerably longer period than might be supposed from the advertising evidence alone. This said, it seems almost certain that manufacture of the "classic" Frog engines (including the Frog 80 Mk III) had ceased by the end of 1970.

In the early 1970's, the Frog name made a brief reappearance in the form of an engine known as the Frog Venom .049, reviving a name that had been used by IMA back in 1961 for the glow-plug version of their outstanding but regrettably short-lived Viper 1.5 cc diesel. However, this was simply a red-headed radial mount version of the then-new D-C Wasp with an alternative crankcase casting bearing the Frog name. It was in fact a straightforward case of badge engineering-a tarted-up D-C Wasp with the Frog name attached. This variant was marketed for a few years in the early 1970's, but the Frog name appears to have finally disappeared from the model engine marketplace by 1974.

So ended the final chapter in the 43-year story of a proud name in British model engine history. It's a great shame that it had to end that way, but a combination of changing market conditions and a failure by management, or perhaps a simple lack of the necessary resources, to respond appropriately to these changes had made the end inevitable. It's a sad fact that far too many pioneering British model engine ranges met a similar fate.


The Frog 50 was made in far smaller numbers than the later 80 diesel models and their survival rate was also somewhat lower. Consequently both the Mk I and Mk II variants of that engine are relatively rare today, albeit well worth looking out for. Understandably, these engines tend to command higher prices than their later descendants apart from the similarly-rare .049.

The Frog 0.8 cc engines are sturdy and dependable little powerplants which are still well able to offer good service to any owner interested in flying "classic" model aircraft. The Mk II and Mk III diesel versions are particularly to be recommended if they are to be used for actual flying.

However, all variants of the Frog 80 series are fine collectibles in their own right. The diesel models were made in sufficiently large numbers that good examples remain readily available today (2010) on eBay and elsewhere at by no means prohibitive prices by present-day standards. The .049 glow-plug models were manufactured in far smaller numbers and hence are significantly harder to track down, but they are out there (and more expensive when you succeed!

Any of the engines described in this article is well worth adding to any collection of British model engines from the "classic" era. So go ahead—keep your eyes open and grab one if it comes up! You won't regret it.



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