The engine was a longitudinally mounted inline vertical OHC 500 cc twin based on an experimental 1932 BSA design (the Line-Ahead-Twin - LAT) with coil ignition and wet sump lubrication which, through a dry clutch, drove a shaft drive to the rear wheel.

This had a much higher compression ratio with a different OHC design but was never sold, reputedly because of the undampened front fork system which affected handling.

Serious problems with vibration made the new Sunbeam bikes uncomfortable to ride and the initial production originally sent to South Africa was recalled.

[2] The in-line engine was inherently suitable for shaft drive, and BMW-style bevel gears would have been ideal.

However, having inherited some worm-gear machinery from the Lanchester Motor Company, BSA opted to specify worm drive rather than bevel gears.

When Sunbeam production ended BSA sold the remaining stock of parts to Stewart Engineering.