BSA B31

 

 

 

 

Make Model

BSA B31

Year

1945 - 59

Engine

Single, OHV, 4-stroke

Capacity

348 cc / 21.2 cub in.

Bore and Stroke

71 x 88 mm

Compression Ratio

6.5:1

Cooling System

Air cooled

Lubrication

Dry sump

Exhaust

Single, stainless steel and chrome

Carburetor

Single Amal type 76

Ignition 

Lucas magneto MO1L

Starting

Kick start

Clutch

Multi-plate

Maximum Power

16.2 kW / 22 hp @ 6000 rpm

Transmission 

4-Speed manual

Final Drive

Chain

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

Colours

Green/chrome plated tank and black frame /Maroon

Source

Wikipedia

 

 

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 The model continued in production until 1959, by which time the traditional Lucas magdyno had been replaced by an alternator and coil ignition.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.