From the MEWS archive, this model was photographed at a Model Engineer exhibition. It is a half-size 600cc BSA M21 side valve engine dating from the 1930s - 60s. The motorcycle fitted with a sidecar became well-known as the vehicle used by the Automobile Association as a mini mobile workshop for members’ roadside repairs.

The M21 was a larger version of the ubiquitous 500cc M20, as used by the British Army during World War II. Production was interrupted when the factory was bombed causing severe damage and loss of life.

The M21 initially had a bore and stroke of 85mm x 105mm for a swept volume of 595cc. In 1938 it was brought into line with the M20 with a bore of 82mm and a 112mm stroke giving 591cc.

The engines were designed by Valentine (Val) Page, the JAP and Ariel designer who was also responsible for the Triumph 6/1 and BSA Gold Star.