bsa motorcycles - 1915 BSA K






The ubiquitous BSA side-valve! What a proud boast for a basic, primitive design that is seen everywhere and taken very much for granted. The BSA M20 is the more common garden variety BSA side-valve; but how much did the 600 vary from the 500 and just what is it like to ride the much loved, much abused and much respected BSA slogger?

The M21 tested is a product of the immediate post-war motorcycling industry in that it shares the running gear of the war model WM20 despite being a claimed 1947 model. The pre-war appearance is the result of most motorcycle factories producing interim civilian models based on pre-war/war model designs before releasing their new post-war range in late 1947 and 1948.

First though, a history lesson and then we ride the bike. Pleasure must accompany pain, although I'm not sure which process is the most painful. Again I'll assume that the reader is unfamiliar with the model, please bear with me all of you BSA side-valve fanatics.

The performance of the M21 was claimed to be a solid, pumping 15 BHP, however this did not translate to top speed, with sidecar gearing 50 mph was a less than comfortable maximum. By way of comparison the M20 with solo gearing was good for 13 torquey BHP that guaranteed a good 55-60 mph.

The side valve’s low revs, the long inlet tract, low compression and the poor thermal efficiency of the combustion chamber all contributed to the low power output but then a high top speed wasn't the intention.The M21 initially had a bore and stroke of 85mm x 105mm for a swept volume of 595cc. In 1938 it was brought into line with the M20 with a bore of 82mm and a 112mm stroke giving 591cc. A similar bore as the M20 turned the M21 into a long stroke version of the 500 and meant the factory could use common engine parts. The M21 still used heavier flywheels though, to provide the low rev pulling power needed for sidecar work.

Detail changes to the BSA "M" series were slight before the dreaded "Hun" started the "big one." The popular M21 side-valve was first produced in 1937 accompanied by a range of "M" series BSA's. There was the M19 350cc OHV Sports, a solo M20 500cc tourer, the 600cc M21 for sidecar work, the M22 500cc OHV tourer and the M23 "Empire Star". This bewildering array of models did little to help BSA's marketing strategies and the company was forced to rationalise its line-up, even if only to help with its spares situation.

The military orders came flooding in. The War Department took every machine the BSA factory could make including M21's. After the fall of Dunkirk the supply situation was so grim all efforts were concentrated on churning out M20s, despite their clear unsuitability for military use. War is hell and the military was determined that it wouldn't only be the enemy that suffered. For off-road use the BSA was overweight, had cumbersome handling, precious little ground clearance and was somewhat pedestrian beside the envy generating Matchie G3/L. As a trade-off and I shouldn't be too harsh, the BSA M20 was well made, reliable and mechanically sound and relatively quiet.



Photo from kind permission of the Motorbike Search Engine

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