The M21 from British manufacturer BSA sports an air-cooled, four-stroke, 591cc, single cylinder powerplant mated to a four-speed manual transmission that can reach a maximum power output of 15 horsepower at 4200 rpm. Standard fittings for this classic machine include laced wheels, drum brakes in the front and in the rear, a large headlight, a hard-tail rear suspension, a single spring girder fork in the front, a sprung single seat and a chromed exhaust system.