Make Model



1945 - 59


Four stroke, single cylinder, Overhead valve


348 cc / 21.2 cub in.

Bore and Stroke

71 x 88 mm

Compression Ratio


Cooling System

Air cooled


Dry sump


Single, stainless steel and chrome


Single Amal type 76


Lucas magneto MO1L


Kick start



Maximum Power

16.2 kW / 22 hp @ 6000 rpm


4-Speed manual

Final Drive


Front Wheel

WM2 x 19 in.

Rear Wheel

WM2 x 19 in.

Front Tyre

3.25 x 19 in. Dunlop Gold Seal

Rear Tyre

3.5 x 19 in. Dunlop Gold Seal

Front Suspension

Hydraulic fork

Front Suspension

Hydraulic fork

Rear Suspension

Swinging arm fork with shocks

Front Brake

8 in., drum

Rear Brake

7 in., drum

Wet Weight

175 kg / 385 lbs

Fuel Capacity

19 L / 5 US gal

Top Speed

110 km/h / 70 mph


The BSA B31, introduced in 1945, was the first new model introduced by the company after the Second World War. Based on pre-war designs, it used a single cylinder four stroke engine that displaced 348 cc (21.2 cu in). Initially, it had a rigid frame and telescopic forks, the first use of such on a BSA. It developed about 17 bhp (13 kW), adequate for the roads of the day and enough to deliver a top speed of around 70 mph (110 km/h). It was immediately popular and was soon joined by a 500 cc (31 cu in) version, the BSA B33 and competition equivalents, the BSA B32 and BSA B34; the side-car version, with stiffer suspension and different final demultiplication, was known as BB31.

Plunger rear suspension was offered later, with a swingarm rear suspension frame available from 1954. The model continued in production until 1959, by which time the traditional Lucas magdyno had been replaced by an alternator and coil ignition.

The B series expanded through its life to include the famous BSA Gold Stars, and the bottom half of all engines has much in common with the M series side valve models. The M33, designed for sidecar work, combined the strong M series frame and the better-performing B33 500 cc overhead valve engine.