| MODEL | BSA AA (M20 & M21) 1955 - 1961 |
| Year | 1955 - 1961 |
| Engine Type | Four stroke, single cylinder, side valve, air-cooled |
| Displacement / Capacity | 591 cc / 36.1 cu in |
| Bore X Stroke | 82 x 112 mm (85 x 105 mm up to 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 |
| Front Suspension | Telescopic fork (girder up to 1947) |
| Rear Suspension | Plunger (none up to 1947) |
| Front Brakes | Drum, 8 in (7 in up to 1955) |
| 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 | — |
| Wheelbase | — |
| Ground Clearance | — |
| Dry Weight | 167 kg / 369 lbs |
| Wet Weight | — |
| WHEEL SPECS | |
| Front Tyre | 3.25 x 19 |
| Rear Tyre | 3.50 x 19 |
| FUEL SYSTEM | |
| Fuel Capacity | 11.4 Litres / 3 US gal |
| Fuel Consumption | — |
| Top Speed | — |
Photo: sv1ambo, CC BY 2.0, via Wikimedia Commons
Photo: User Arpingstone on en.wikipedia, Public domain, via Wikimedia Commons
The side-valve M20/M21 single in the form used by the Automobile Association's road patrols, usually with a sidecar. The AA ordered more than 2,000, with stronger front brakes, a 12 V system for radios and Western-style bars, and was the last customer for the M21 before production ended.
