circa 1939, 500cc, single-cylinder sidevalve


Classic bike buyers and riders are so used to looking at military girder-forked 500cc BSA M20s and 600cc M21s that they forget just how attractive these bikes can look in civilian trim. This example is not original, and that kickstart lever looks to be in the wrong position for a decent swing-through. But clearly someone has put a lot of effort into building this softly-spoken, single cylinder sidevalve.





1939 BSA WM20 specifications

Type: Air-cooled sidevalve single
Capacity: 496cc (500cc)
Bore & Stroke: 82mm x 94mm
BHP: 13 @ 4200rpm
Compression ratio: 5:1

Carburettor: Amal 276
Transmission: 4-speed, multi-plate
wet clutch
Brakes: 7-inch drums front and rear
Electrics: 6-volt, magdyno
Front suspension: Girder
Rear suspension: Rigid
Wheels: 3.25 x 19-inch front & rear
Weight: 369lbs (dry)
Maximum speed: 55mph (approximately)




Top BSA M20 & M21
specialists and links


The BSA M20 website

Probably the best military M20 site in the world owned and managed by Dutch
BSA M20 expert, Henk Joore.


Ian Wright at Ark Motorcycles

Britain's top M20 & M21 expert, and
hugely knowledgeable on all M-series
and B-series BSAs. Talk to Ian about
other military matters too.

UPDATE: Ian Wright has retired.


BSA UK Owners Club


Vintage Motor Cycle Club



Ariel and BSA specialists. Hugely knowledgeable. Remanufactured

and hard to get spares.


circa 1939 500cc BSA M20



Pre-war BSA M20s don't come up for sale that often, and certainly not at this price. Bonhams sold this BSA M20 in April 2013 for just £2,760 (including buyers premium).


Normally, we would have expected the price for a machine in this condition to be around £3,500, or even slightly higher. But there were two particular factors that no doubt helped depress the price.


Firstly, there were no documents with the machine. That wouldn't have bothered us too much, personally. Registering a bike in the UK isn't particularly difficult, or expensive. Just a little time consuming. But an unregistered bike does put the provenance into a questionable light. And (second factor) the engine and frame numbers are way apart.


For all that, it is an attractive machine (albeit in the wrong shade of BSA green), and it appears to be correct in all the major details such as the Lucas DU142 8-inch headlight (as opposed to the later WM20 DU42 6-inch headlight used from 1942 onward for the duration of the war ).


The rear mudguard is valanced, which is correct for an M20 or M21 of this era. But the fork links and fork damper would not have been chrome plated.


The frame number of this bike is WM20 5045. The engine number is ZM20 7475 which, it seems, dates it to March 1951 (went to Paris). The gearbox is post-1944. The fuel tank is post-war. Ditto the primary chaincase.


That aluminium alloy cylinder head, incidentally, should not be on a pre-war bike (if correctness is important to you). Aluminium alloy heads (with a brass spark plug insert) didn't appear until 1951. The cylinder heads on all earlier bikes would have been cast iron.


Neither would it have had a centre stand. That comes from a 1945-1954 plunger model.


Regardless, the bike is not purporting to be anything other than a circa 1939 M20, and it more or less looks the part—and we've got no doubt that you can have as much (or as little) fun on this as on any other BSA sidevalve.




Back to Sump's main BSA M20 page



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