By Royce A Limb

How it all began. .

Having enjoyed working on many engines over the years planted the idea in mind to one day have a go at building a 'one-off' of some sort. Having successfully built a 4" scale McLaren traction engine after retiring provided the impetus to look at various designs and types, coming across one particular publication detailing a scale design of this unusual engine, subsequently modelled by Doug Kelly. It has to be said I was inspired by both the book and the fine model built by Doug; however, considering the dimensions, I couldn't help wondering if it could be scaled up to produce a more substantial model, but as many will know this is not always as straight forward as it sounds so armed with the original book as a loose guide, I proceeded…

Certain items and materials were sought and purchased quickly passing the point of no return having invested so much. Working to this scale had certain advantages in that basic cylinder castings came 'off-the-shelf’ from the likes of Westwood Liners; similarly bevel gears from HPC Gears, standard 10mm spark plugs and big end bearings matching a standard needle roller size which can easily be grease lubricated. The distributor is a second-hand ex Fiat 600 unit fully refurbished. The fuel/air mix is handled by an ex Austin A35 carburettor.

The bore and stroke are 2 and 4 inches respectively giving a displacement circa 900cc. With hindsight I should have increased the compression ratio from the 4:1 as recommended in the book as I feel this would have improved ignition/firing greatly. Significant work went into redesigning items to aid construction and manufacture of components easier in this scale however in some locations the fasteners required were a close fit. The other implications of larger models is one of cost which appears to increase disproportionately at times!!

The twin double-acting engine is in fact a 'four' as it produces four power impulses every two revolutions of the flywheel. The cylinders of my model are fitted with loose coolant jackets sealed at each end by O-rings and the corresponding heads all benefit from full flow coolant circulation provided by centrifugal pump. The pistons carry twin ring sets as per the original publication. The piston rod is carried in four long cast iron sleeves sealed at four points by piston rings.

The valves are 'caged' as per large engine design practice, likewise the cylinder heads are sealed with copper rings. The caged valve design provides ease of maintenance and service. The rocker arms and flywheel were cast by Ballantine Castings of Bo'ness, Scotland to precision patterns produced by a very talented friend. The post-build addition of the functioning air-start system has proved reasonably successful and adds a little more realism to this model which remains notoriously difficult to start and may yet benefit from further


Despite early misgivings and problems along the way I have enjoyed the work and challenge of building this model and I am constantly encouraged by the words of Henry Royce,

"Whatever is rightly done, however humble, is noble"

Finally, it is worth mentioning neither my model or, I believe, the one detailed In the book should necessarily be considered as an exact replica of this fine engine given no works drawings or details are available but may serve as a fair representation of the type in respect to the Snow Engine Company providing a working insight into this unusual and powerful engine.

Photos from the Midlands Model Engineering Exhibition 2019.