The Atom Minor Mk III is one of a long series of "Atom" designs by the late Edgar T Westbury, intended for home construction. The first Atoms (designated I through V) were large two-strokes, single cylinder engines ranging from 52 down to 30cc. The Atom Minor Mk I was designed in 1932 and detailed in the Model Engineer, volume 67, issue 1640. This was a 0.89 cuin (14.6cc), plain bearing, side-port, spark ignition engine. Westbury considered it to be the first engine to demonstrate the possibilities of reasonably small power-driven model aircraft. The engine, complete with airscrew, weighed 17.5 ounces.
A later edition revision of the Atom Minor Mk I included ball races for the crankshaft journal bearings and was in production for some time by Messrs AE Jones Ltd. Responding to requests for a smaller engine, Westbury reduced the displacement to 6cc while retaining the main characteristics of the original Atom Minor, using a combination of plain bearing with a single ball race at the crankweb end of the shaft. This engine, effectively the Mk II version, was called the New Atom Minor.
The Mk III appeared in 1947 as a 0.355 cuin (5.8cc) two-stroke, spark ignition, plain bearing, single cylinder engine with rear rotary valve induction. Besides the revised inlet porting scheme, the Mk III dropped the integral fuel tank and backplate mounting in favor of the more "modern" beam mounting. Unlike its elder brother, it was not serialized in the Model Engineer. Instead, the ME publisher of the time, Percival Marshall Ltd, offered a booklet detailing the engine and its construction. At the same time, they produced a single sheet, full size plan (PE13) for those content with just the music.
In 1984, the booklet was reproduced by TEE Publishing Ltd (UK), who sadly omitted the prior publishing history, but added the not very helpful note on their copyright page to the effect that the Atom Minor had been designed in 1933. As we have seen, the year is wrong, and the connection between the 5.8cc 1947 Mk III and the 14.6cc 1932 Mk I engine lies mostly in the name and designer, although a certain family heritage is evident.
Castings are essential for a prospective Atom Minor Mk III builder. For years, green sand castings were marketed by a company named "Woking Precision" (UK). Upon examining their castings, more than one buyer of Woking's castings for the Atom and other Westbury designs made light of the "Precision" part of the name. Today (2012), castings, plans, and the booklet are sold by Hemingway Kits (UK). Their sand cast products use the original patterns, so some fettling is required unless you can live with front and rear covers which merely approximate the profile of the main casting to which they attach (look for the fettling in the images at the head of this page). Still, the Atom Minor set is not as awful as some of Woking's other efforts and an untold number of builders have used them to produce fine running examples of the engine.
Plans and Booklet
As mentioned in the Background, both the plan and construction booklet remain available. The booklet contains full size drawings of each part together with very complete construction notes. ETW has written these in a sequence designed to allow the builder to correct for minor mistakes in hitting the numbers on parts made earlier. In our review of this booklet (see Model Engine News, April 2012), we noted two main things:
- The techniques and set-ups described are just as fundamental to model engine building today as they were back in 1947. As such, this is a valuable book for beginning engine builders, even if they do not make an Atom Minor Mk III.
- There is an error in both the booklet and the original PE13 plan relating to the between centers distance for the connecting rod (p33). In the booklet, it is shown as 1-43/64" (1.6719). The correct dimension is more like 1.703" (1-45/64"). However, if the booklet text is followed, this error is of no consequence (see Model Engine News, May, 2012).
At one time, this error was repeated on the PE13 plan, however we understand that Hemingway have corrected the master and modern copies of the plan show the correct length. Not much they can do about TEE's booklet though.
Here are the machined components for the Atom Minor Mk III, as made by Jan Huning (UK) from Hemingway parts and plans. The engine uses cast iron for the cylinder, piston, and rotary valve. The crankshaft is a rather unusual and delicate affair, with a journal diameter of a mere 0.250" for its entire length. The prop driver incorporates a tapered collet designed to grip both the prop hub and crankshaft, but to allow slippage in a sudden stop. With a 1/4" diameter shaft, this may be no bad idea, and has it's genesis in Westbury's boating heritage where a sudden dunking would produce an instant stop of the engine due to hydraulic lock. The flywheel however would still contain considerable kinetic energy, so if shaft and flywheel were keyed, a bent rod was almost guaranteed, hence ETW's preference for a friction fit.
Several aspects of the engine show ETW's affection for "full size" practice, such as the rounded ends and roots of the cooling fins, and the "I" beam cross-section of the connecting rod. These features, and the setups required to position the castings for machining make the Atom Minor Mk III a real "builders" engine—one for those who delight in detail and craftsmanship. The end result will more than adequately reward the time spent.
For a blow by blow description with photos of how Jan machined his Mk II, follow this link to Building the Atom Minor Mk III.
Here's Jan's engine, completed Easter, 2012, running under spark ignition. The timer uses an insulated fiber "plunger" and a flat (an arc really) machined on the rear of the prop hub to drop the moving point into contact with the fixed at somewhere near TDC, depending on the position of the timing arm. As we have already seen, the hub is not keyed to the shaft in any way, so a certain amount of dancing is required while tightening up the prop hub assembly, as the timer position will depend on this. In the picture, Jan's engine is running under spark ignition using a mixture of 78% unleaded petrol with 22% two-stroke motorcycle oil.
The engine, as dimensioned, is also quite happy with glow plug ignition. There have been at least two articles published which detail a higher compression head deemed necessary by their authors to accomplish this feat. However, the image here of the engine built by Brian Cox (France), shows a totally stock Atom Minor Mk III with a glow plug in place of the 1/4-32 spark plug, happily defying the experts.
By modern standards, ETW's Atom Minor Mk III is large and heavy for an almost 6cc engine. But remember it was designed before the glow plug era, and embodies more full-size motor cycle engine concepts than a modern engine of the same class. It has a certain period appeal to it and any builder of the engine should be proud of their efforts.