Classic British Bike Magazine

BSA M20 & M21

500cc & 600cc. 1937-1963. Single cylinder air-cooled sidevalve review


BSA M20 & M21 Pros


Mechanicals: Simple, but some head-scratching required.

Handling: Surprisingly good. You can throw these around.

Forks: Tele models are predictable. Girder bikes are crude.

Starting: Sorted bikes fire-up with a gentle nod and a prod.

Spares: Still enough parts around to keep them rolling.

MOT: Not required any more. No road tax required either.

Looks:. Classic Brit single appeal. Olde worlde charm.

Sound: BSA M20/M21 sidevalves chuff very satisfyingly.

Clutch: Fairly light and smooth in action.

Resale: Military bikes in high demand. Less so civvy iron.

Magneto ignition: Separates the sparks from the electrics.

Re-enactment: Dress like a despatch rider. Fun for some.

Investment: WM20 military bikes hold their values well.

Brakes: Later post-war M21s have good front stoppers.

Tough: Solidly built. All steel and cast aluminium.

Engine: Strong. Exhaust valves need care and love.


BSA M20 & M21 Cons


Economy: Poor. 40-45mpg at cruising speeds.

Oil: Most leak a little from one or more joints. Manageable.

Performance: 45-50mph cruising. Flat out at 60. Maybe.

Brakes: Generally poor, but careful set-up helps.

Acceleration: Hopeless. 0-50mph eventually.

Maintenance: Regular lovelorn tinkering required.

Tools: Whitworth spanners. Some special items needed.

Comfort: Pillions will hate it. Riders will merely endure.

Lighting: 6-volt electrics. Feeble all round.

Gearbox: Prone to leaks. Demands slow changes.

Carburettor: Standard Amal 276 likes to sneeze fuel.

Sidestand: Not on WW2 or pre-war bikes.

Horn: Altettes need good set-up. Otherwise feeble toot.

Sparks: Learn to set ignition points. Not too hard.

Storage: They wet-sump and prefer daily exercise.

[Okay, tell me more about BSA M20s/M21s...]


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