BSA M21

 

 

 

 

Make Model

BSA M21

Year

1937 - 61

Engine

Four stroke, single cylinder, side valve,

Capacity

591 cc / 36.1 in

Bore x Stroke

Up to 1938: 85 x 105 mm

After 1938:  82 x 112 mm

Cooling System Air cooled
Compression Ratio 5.0:1
Oil Capacity 3.4 L / 7.2 US pints
Carburetor 1937-38: Amal 76, 1939-45: Amal 276, 1945 onward: Amal monobloc
Exhaust Single, chrome

Induction

Aspirated

Ignition 

Magdyno
Battery 6 V (except for AA models: 12 V)
Starting Kick

Max Power

11 kW / 15 hp @ 4200 rpm
Clutch Multi-plate, dry

Transmission 

4-Speed
Final Drive Chain

Gear Ratios

Solo: 1st 14.3 / 2nd 9.9 / 3rd 6.3 / 4th 4.8:1

Sidecar model: 1st 16.6 / 2nd 11.5 / 3rd 7.3 / 4th 5.6:1

Frame Twin cradle with integral sidecar lugs

Front Suspension

Up to 1947: Girder

After 1947:  Telefork

Rear Suspension

Up to 1947: None, spring loaded saddle

After 1947:  Plunger type

Front Brakes

Up to 1955: Drum, 7 in

After 1956:  Drum, 8 in

Rear Brakes

Drum, 7 in
Wheels Steel, laced spokes

Front Tyre

3.25 x 19 in

Rear Tyre

3.50 x 19 in

Dimensions

Length:  2180 mm / 85.8 m

Width:     740 mm / 29.1 in

Wheelbase

1372 mm . 54 in.

Ground Clearance

117 mm / 4.6 in.

Saddle Height

724 m / 28.5 in.

Dry Weight

167 kg / 369 lbs

Fuel Capacity 

13.6 L / 3.6 US gal