Make Model



1937 - 39


Four stroke, single cylinder, Overhead valve


249 cc / 15.2 cub in
Carburetor Amal
Oil capacity 2.3 L / 4.8 US pints
Exhaust Single, chrome silencer
Ignition Magneto
Battery 6V
Starting Kick starter


4-Speed, hand change

Final Drive


Gear Ratios

1st 18.2 / 2nd 12.4 / 3rd 8.1 / 4th 6.2


Triangulated cradle type

Front Suspension

Girder, spring loaded fork
Rear Suspension None, spring loaded saddle


1575 mm / 62 in.

Ground Clearance

124 mm . 4.9 in.

Fuel Capacity

13.6 L / 3.6 US gal

In the 1930s Birmingham's BSA was the world's most productive manufacturer, with a stretch of models between 150cc and 1000cc, produced in single cylinder and/or vee twin configurations. They enjoyed a particular strength in the quarter litre class, with a variety of side- and overhead valve models, offered in standard or de-luxe spec, one of the latter's options being foot-change gearbox operation.

Due to a disastrous attempt at the TT, back in the early 1920s, BSA deliberately avoided any forms of competition for the greater part of the Thirties, in complete contrast to their post war philosophy. Even so, in a contemporary magazine road test impression the B21 in question was reported to have 'peppy' performance and, "...contrary to its sturdy appearance was found surprisingly easy to manoeuvre on to the rear stand".