Make Model

BSA B40 / WD40




Four stroke, single cylinder, Overhead valve


343 cc / 20.9 cub in

Bore and Stroke

79 x 70 mm

Compression Ratio


Oil capacity

2.8 L / 5.9 US pints


Air cooled


Dry sump
Carburetor Amal 396, 397 or 398
Exhaust Single
Ignition Magneto
Battery 12V, 8Ah
Spark Plug N4
Starting Kick starter
Clutch Multi-plate with cork insert

Max Power

14.7 kW / 20 hp @ 7000 rpm


4-Speed, foot gearshift

Final Drive


Gear Ratios

1st 12.3 / 2nd 9.6 / 3rd 6.9 / 4th 5.8:1


Full cradle of welded tubular construction


Length: 2100mm / 82.6 in.
Width: 750 mm / 29.5 in.
Height: 1080 mm / 42.5 in.

Wheel Base

1360 mm / 53.5 in.

Ground Clearance

159 mm / 6.3 in.

Seat Height

813 mm / 32 in.

Steering head angle


Turning Circle

4.4 m / 14.5 ft

Front Suspension

telescopic, coil spring, hydraulically damped
Rear Suspension Swinging fork, coil spring, hydraulically damped units

Front Wheel Travel

127 mm / 5 in.

Rear Wheel Travel

70 mm / 2.75 in.

Front Tyre

3.00 x 18, Trials universal

Rear Tyre

3.50 x 18, Trials universal

Front Brake

1.125 x 7 in., mechanical

Rear Brake

1.125 x 7 in., mechanical

Wet Weight

165 kg / 364 lbs

Fuel Capacity

15.9 L / 4.2 US gal

Average Speed

Surfaced road: 88 km/h / 55 mph

Cross country:  40 km/h / 25 mph

Average Range (@ 64 km/h / 40 mph)

322 km / 200 miles

Average Consumption (@ 64 km/h / 40 mph)

4.5 L/100 km / 22.2  km/l / 52.2 US mpg

These machines were the early post war replacements for the remaining fleet of BSA M20's and Matchless GL3's which, together with the US built machines, had formed the major component of the Army's wartime motorcycle holdings.

The B40 with telescopic front forks was a modern motorcycle in its day and was used for convoy control, messenger services and the carriage of any small stores, which could be fitted into the pannier bags. The carriage of messages was an important function at a time when radios were few, and less reliable than now. Shepherding road convoys, particularly on major exercises, with the British Army of the Rhine in Germany, was a vital function. Motorcycles ensured that units did not become muddled, as there was often a set time allocation for the use of roads by military traffic with no allowances for losing the way.

The B40 like most military motorcycles of the period was basically a road machine with limited off road performance.

It is suggested that the total number supplied to the British Armed forces was 3,087, which included 141 for the RAF and 34 for the Royal Navy. BSA received a further order for an additional 235 machines in 1971, which the company was unable to fulfil due to its increasingly precarious state at the time.

As well as serving in the British Army, Royal Navy and Royal Air Force, the WD B40 was sold to the armies of Australia, Denmark, Jordan to name a few.