The Sugden Special



Name Sugden Special Designer David Sugden
Type Compression Ignition Capacity 2.49cc
Bore 0.568" Stroke 0.600"
Production run (home built) Country of Origin England
Photo by (various) Year of publication 1954




The name Sugden Special will be best remembered in England and the British Commonwealth coutries due to the construction article that was serialized in The Aeromodeller, starting with the Christmas issue of December 1954 [1]. The cover of this issue carried a lovely painting by C Rupert Moore of the DH "Mossie". Inside was the plan for a C/L scale model of the aircraft powered by two ED Racers.

Also inside was the start of a lengthy series by Dave Sugden describing (eventually) the construction of his fully developed, high-performance 2.5cc, plain bearing diesel. The series was very well thought out. It began by describing the basic equipment required, and the techniques involved, including back-yard sand casting and pattern making. Along the way, Sugden described the design and construction options considered, and how equipment available influenced design choices.

The series culminated in the June 1955 issue with step by step construction details and the usual reduced size magazine plan. Back then a full size copy could be obtained through the Aeromodeller Plan Service, along with a casting for the crankcase. Even as I write this fifty years later in July 2005, the plan and a crankcase casting can still be obtained (see below).

Essentially the same text and photos were later reprinted in Ron Moulton's Engine Encyclopedia [2]. This book is an excellent compilation of engine related material from pages of the Aeromodeller. It was first published in 1958 and reprinted at least five times, with minor revisions. Sadly, it is long out of print, but can still occasionally be located through sellers on the Internet. I rather suspected that Ron Moulton's editorial hand may have be visible under the excellent, compact prose of the Sugden series. I put this question to Ron and got the following response: [4]

I may well have edited the text at the time (long ago!) but all the copy written around the Sugden Special originated from Dave. He was making the engines in the St. Albans Clubroom in between work at Hatfield with de Havilland.

As the engines began to rival ED and Oliver in performance, I encouraged Dave to provide drawings and a stage by stage description. The reaction exceeded all expectations. We had sand castings made by the sackfull and they went to Tech Colleges, individual model engineers, and apprentice schools--as well as to aeromodellers. They were often an embarrassment as many expected a partially machined crankcase for only 8/- and the actual quality was awful.

Dave migrated to Canada and became a Dental Surgeon. He was several times on the Canadian Free Flight Power Team.

Along with the conclusion of the construction series, the July 1955 issue of the Aeromodeller subjected the Sugden Special to the standard series of tests for "production" engines using their custom made eddy-current dynamometer. The test found it to:

"...conform almost exactly to the expected output of top-class racing engines of 2.5 c.c. size, whilst its power/weight ratio is appreciably better than average." [3]

We'll examine how this was achieved in the Design section.

Plans and Castings

Over the years, the ownership of the Sugden plan copyright changed from MAP to ARGUS, then to NEXUS. The current owner is also the custodian of the venerable Aeromodeller, Model Aircraft, Model Maker, Model Cars, Model Boats, RCM&E, etc, etc plans. For historic reasons, these are known as the X-List Plans. The Sugden Special is plan U588. Click this link to get ordering details, or see the August 2005 issue of Model Engine News for some history of the X List.

A notation below the plan in both the Aeromodeller (see Part 7) and Engine Encyclopedia advertise the plan for 4/6, and a "die-cast crankcase ready for machining" for 8/- (old money). I think we can conclude from the remarks by Ron Moulton [4] that these cases were actually sand castings.

These photos show what are believed to be examples of the "Genuine" Aeromodeller Sugden castings (photos provided by English Motor Boys Ken Croft and Eric Offen). As Ken tells it, a bag of these castings found their way to TEE Publishing as part of an estate sale, and from there to new homes (don't bother to ask; they are all long gone).

However a modern-day sand casting of significantly better quality is available from Canadian Motor Boy, Andrew Coholic. The photos here show his split pattern for the case, and a raw casting straight from the petrobond sand, and afre removal from the casting gate. Note that his split is in the conventional vertical plane, unlike the unconventional and problematic horizontal plane split shown in Part 2 of the construction series.

But there was a gravity die-cast case made for the Sugden by Dunham Engineering (Dunham has made some nice reproduction engines). Australian Motor Boy David Owen has one--someplace (keep looking Dave, we need the photo for completeness). Remarkable how many of us have castings, but have not built the engine.


The engine design had been refined by Dave Sugden over the period prior to 1955--notice that Part One of the Aeromodeller series shows a photograph of six prototypes, including a ball-race glowplug version. Design-wise, it is a standard plain bearing diesel with a bore of 0.568" and a stroke of 0.600" for a displacement of 2.49cc.

It employs what was then "state of the art" porting which extended the transfer duration over that achievable with the "peripheral porting" common in English made engines of the time, such as those made by FROG, Davies-Charlton, Electronic Developments (ED), and others. In these, the transfer ports and exhaust ports were typically cut one above the other with a slitting saw. This meant that porting was a compromise between obtaining adequate transfer duration without producing excessive early exhaust opening.

The Sugden's ports were based on the Oliver Tiger series (pictured here). These engines, which were sweeping the competition circuits, drilled the transfer ports to enter the liner at an upwards angle in the area that separated the exhaust slits (the Owen Mate uses the same scheme). This allowed the transfer to open much sooner after the exhaust than other English engines of that time. The concept, which has become known as "Oliver porting", was widely copied by other designers.

The method chosen to retain the cylinder liner is worth a comment. The liner drops into the case and is secured by a "clamp ring" (seen here) and four 6BA screws (or 4-40 if you prefer). The cooling jacket screws onto a fine 40 TPI thread of the protruding section of the cylinder. Some similar arrangements had been used on the Yulon, early AMs, and others, but Dave's approach is rather unique. It side-steps the difficulty of fitting nuts to studs suffered by the early AMs, and the weight and other problems associated with long hold-down bolts of the later AMs, ED Racer, and the Olivers (they need to be tough to prevent failure in torsion during tightening, so will be relatively heavy). The screwed on cooling fins also provide better thermal conductivity than a slip fit head.

Two of Dave Sugden's stated design goals had been light weight and simplicity of construction. These he achieved by very careful of unnecessary, "parasitic" material, coupled with a plain bearing crankshaft. Although long in the front (presumably for streamlining in the nose of a team racer), the case carries a short bronze bushing that terminates just ahead of the venturi. Forward of this, an open space acts as a lubrication reservoir. The very front of the shaft runs unbushed in a reamed section of the case itself.

The piston height is very short, as is the cylinder liner and the conrod. This produces an engine which is very compact vertically. It also results in the piston skirt raising completely above the exhaust at TDC. This opens the case to atmospheric pressure, providing a degree of extra air to enter the case, allowing a heavier weight of charge to be inducted while retaining a comparitively small inlet diameter to assist venturi effect, and hence fuel draw and atomization. The technique is widely known as Sub-piston Induction. The photo here shows how at TDC, only the conrod is visible through the exhaust ports! Examination of other racing engines such as the Oliver Tiger, DC Rapier, McCoy 60, etc show that 10 to 15 thou of SPI is not uncommon. That exhibited by the Suden is most uncommon; the only other commercial diesel engines I've seen with this feature being the AM25 and AM35.

Weight wise, the Sugden comes in at a low 124 grams. Compare this with the competition of the time: a magnesium case Mk II ED Racer weighs 158 grams; an Oliver Tiger 178. Removing the bulkhead mounting lugs would save even more weight, so we can say that Dave Sugden amply achieved his low weight goal.

Builders' Notes

When this page was first written, I was unable to get a photo of a real, finished, running Sugden. The only solution was to build one. This is detailed on the Sugden Construction page. Modern-day Sugden builders would be advised to make some minor changes. First, make a more conventional spray-bar with a needle seat, and fit a DCO contra-piston. Perhaps even loose the radial mounting option feature unless it is really needed.

There is a noticable difference in the venturi between the prototypes and the final version depicted by the plan; the former having an Oliver-like screw-in venturi, permitting the angle of the NVA to be adjusted by shimming. The final Sugden uses a somewhat unusual arrangement where the spray-bar screws into one side of the venturi and is secured on that side by a jam nut. The same thread carries the needle valve, while the fuel inlet side is a slide fit in the hole on the other side of the venturi and a potential source of air leakage. This also prevents the venturi from being turned around for upright or inverted mounting. My advice: loose it. Make a conventional spray bar, or even consider reverting to the Oliver type screw-in venturi design.

For the rest, follow the plans closely; except for the 3/16" reamed hole in the front of the crankcase that should be 5/16" that is. The Sugden is a well developed design that should give very pleasing results with comparitively little effort. It would make a good choice for the builder who have made enough simple engines to get the feel for the precision of the fits required and is ready for something more sophisticated.


Although an old design, the Sugden Special is a good choice for modern builders wanting to try their hand at a relatively simple high performance engine. The original article is also a good place for beginners to get a grasp of the equipment and techniques required to build their own engine. Speaking of which, the method described for turning the crankpin is unusual to say the least. So much so that it has been added to the Crankshafts How-To page.

Part 1, December 1954

Part 2, January 1955

Part 3, February 1955

Part 4, March 1955

Part 5, April 1955

Part 5 (cont)

Part 6, May 1955

Part 7, June 1955

Part 7 (cont)


[1] Sugden, Dave: Making Your Own Engine
Part I
, The Aeromodeller, Model Aeronautical Press Ltd, Watford, England, Volume XIX, Number 227, December 1954, p652.
[2] Moulton, Ron: Engine Encyclopedia, Model & Allied Publications, 1958 (1971 reprint), p138.
[3] ibid. p 172.
[4] Moulton, Ron: Extract from private email correspondence, July 24, 2005.





Please submit all questions and comments to