The BSA C11 was a pre-unit single-cylinder developed before
the Second World War from the sidevalve C10. Fitted with overhead valves and
displacing 249cc, the C11 was launched in 1939 and continued to be developed
into the 1950s. The C11 frame was improved in 1951 when BSA added plunger rear
suspension.
Military W-C11
Although British armed forces only used a few BSA C11s as lightweight transport
(having selected instead the BSA M20) in 1940, 530 C11s were ordered in military
specification by the Office of the High Commission of India. Modifications for
war use included a rear carrier over the mudguard, an additional air filter on
the fuel tank and a mag dyno in place of the coil.
BSA C11G
Problems with the gearboxes were addressed by the model C11G, which was
available either with a three speed gearbox and a rigid frame or with a four
speed gearbox in the plunger frame. The C11G also had improved front brakes and
became popular as a post war commuter motorcycle.
Steve McQueen's C11
Actor Steve McQueen owned a 1951 BSA C11. Painted in British racing green,
McQueen's C11 had a chrome tank with green and gold accents and chrome wheels
with green spokes. After McQueen's death the C11 was kept by his wife Barbara
for many years before being sold by auction.
Source Wikipedia
Any corrections or more information on these motorcycles will be kindly appreciated.