The BSA B50 was a single-cylinder 499 cc (30.5 cu in) ohv motorcycle, produced by BSA at their factory in Small Heath, Birmingham. The last of the big capacity unit-construction singles from the Birmingham Small Arms company, it had an alloy engine with a bore of 84 mm (3.3 in) and a stroke of 90 mm (3.5 in). As well as the road version, special models were produced for off-road competition use.