| MODEL | BSA M21 600 1937 - 1961 |
| Year | 1937 - 1961 |
| Engine Type | Four-stroke, single-cylinder, side-valve |
| Displacement / Capacity | 591 cc / 36.1 cu in |
| Bore X Stroke | 85 x 105 mm (to 1938); 82 x 112 mm (after 1938) |
| Compression Ratio | 5.0:1 |
| Cooling System | Air-cooled |
| Max Power | 11 kW / 15 hp @ 4200 rpm |
| Max Torque | — |
| Ignition | Magdyno |
| Induction | — |
| Start System | Kick |
| TRANSMISSION SPECS | |
| Lubrication | — |
| Gear Box | 4-speed |
| Clutch | Multi-plate, dry |
| Final Drive | Chain |
| CHASSIS SPECS | |
| Frame | Twin cradle with sidecar lugs |
| Front Suspension | Girder (to 1947), Telefork after |
| Rear Suspension | None (to 1947), plunger after |
| Front Brakes | Drum, 7 in (to 1955); 8 in (from 1956) |
| Rear Brakes | Drum, 7 in |
| DIMENSION SPECS | |
| Overall Length | 2180 mm / 85.8 in |
| Overall Width | 740 mm / 29.1 in |
| Overall Height | — |
| Seat Height | 724 mm / 28.5 in |
| Wheelbase | 1372 mm / 54 in |
| Ground Clearance | 117 mm / 4.6 in |
| Dry Weight | 167 kg / 369 lb |
| Wet Weight | — |
| WHEEL SPECS | |
| Front Tyre | 3.25 x 19 in |
| Rear Tyre | 3.50 x 19 in |
| FUEL SYSTEM | |
| Fuel Capacity | 13.6 Litres / 3.6 US gal |
| Fuel Consumption | — |
| Top Speed | — |
The 600 cc big brother of the M20, a long-stroke side-valve single with a bit more pulling power for sidecar and military work. It lasted well into the 1960s and gained telescopic forks and a plunger frame after the war.
