The BSA M20 was just part of BSA’s massive wartime contribution to Britain’s war effort. But it was a very big part. 1937 was its first year of production that would last until the late 1950s. Designed to be simple, rugged and reliable, the M20 was a side-valve (flathead) 500cc air-cooled single with a cast-iron top end. It had a rigid frame and BSA’s pre-war girder front end. Everything on the bike was extra-heavy duty for military use. The M20 was designed by Val Page as a heavy-framed sidecar rig, so the engine had tons of low-end torque. When sent to the British Army for testing (without the sidecar), they didn’t like it saying it was heavy, slow and had poor ground clearance. But it was saved by its toughness, reliability and ease of maintenance and repair. Of course, BSA was the world’s largest motorcycle maker at the time, and also a huge defense contractor, and that probably helped. In the end, the Army, the Royal Navy, and the RAF placed huge orders for the M20, along with Sweden, South Africa and India.
126,000 M20s and WM20s (the military designation) served in nearly every field of battle during World War 2. This makes it the most-produced military motorcycle of the war. The M20 was also one of the longest-serving military motorcycles in British motorcycle history, serving from 1937 through the early 1960s. BSA’s Small Heath factory was bombed by the Germans in 1940, but mega-corporation BSA had 67 other factories scattered around England, so production quickly resumed.