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by Roger Worton

The following is based on information from The Motor Cycle, August 1955;

The Torroidal (radial) 2-stroke BSA cyclemotor engine.

In late 1938, BSA began designing a two-stroke torroidal engine.  And this little unit - it was of 34cc swept volume - was intended as a cyclemotor.  It definitely did run although, by all accounts, a prototype produced little more than sufficient power to overcome its own friction and pumping losses.

BSA Torroidal engine

The cylinder half castings are in cast iron and, since the cylinder revolves, have curved vanes round the periphery to assist cooling.  Dividing the torroid are two partitions at 180 degrees; within each half is a long double-ended piston and two pistons are coupled by a diametrical arm.

Shafts and gearing

On the crankshaft is a gear meshing with another of the same size co-axial with the torroid.  Since the second gear is fixed to the wheel spindle, it follows that the casing drives itself round the spindle.

Mounted on the hub, the carburettor (note Amal type 335 as fitted to conventional Winged Wheels) feeds into the rotating inlet tract; it was not envisaged that the noise of two 17cc cylinders would be excessive.

As the drawing shows, the resultant power unit is far from compact but is not so complicated as it looks.  The design did not get very far because of the difficulty of producing cylinders and pistons.

"Eat your hearts out" NSU and Norton rotary engines.

[I suspect that the fitting of an Amal 335 was from the imagination of the Blue Un's artist, since that carburettor was not available in 1938 - Editor]

First published - June 1998

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