| MODEL | BSA B21 250 1937 - 1939 |
| Year | 1937 - 1939 |
| Engine Type | Four-stroke, single cylinder, OHV |
| Displacement / Capacity | 249 cc / 15.2 cu in |
| Bore X Stroke | — |
| Compression Ratio | — |
| Cooling System | — |
| Max Power | — |
| Max Torque | — |
| Ignition | Magneto |
| Induction | Amal carburettor |
| Start System | Kick starter |
| TRANSMISSION SPECS | |
| Lubrication | — |
| Gear Box | 4-speed, hand change |
| Clutch | — |
| Final Drive | Chain |
| CHASSIS SPECS | |
| Frame | Triangulated cradle type |
| Front Suspension | Girder, spring loaded fork |
| Rear Suspension | None, spring loaded saddle |
| Front Brakes | — |
| Rear Brakes | — |
| DIMENSION SPECS | |
| Overall Length | — |
| Overall Width | — |
| Overall Height | — |
| Seat Height | — |
| Wheelbase | 1575 mm / 62 in |
| Ground Clearance | 124 mm / 4.9 in |
| Dry Weight | — |
| Wet Weight | — |
| WHEEL SPECS | |
| Front Tyre | — |
| Rear Tyre | — |
| FUEL SYSTEM | |
| Fuel Capacity | 13.6 Litres / 3.6 US gal |
| Fuel Consumption | — |
| Top Speed | — |
The 250 cc overhead-valve model in BSA's redesigned 1937 single-cylinder range, with a four-speed hand-change gearbox and a triangulated frame. Period testers found it lively and easy to handle despite its solid build. BSA was largely out of racing at this time, so it was sold as a sound road machine.
