A little off-topic but I’m sure you will forgive me. I’ve got my eye on a 1950’s BSA M21 which the owner believes to be an ex-AA machine although it has lost most of the obvious fittings such as sidecar body, leg shields and fairing. The photos aren’t very clear but at least it has the butterfly type petrol filler cap, which is some indication that it may have been specified for the AA. The engine and frame numbers match, which the owner says is another indication – apparently the AA stamped replacement engines with the same number as the frame. Can anyone verify that statement, and are there other tell-tales that I need to check for when I go to see the bike?

To be honest it looks like a reasonably priced bike anyway, but I'd like to know whether I'm (potentially) buying something with a bit of history.

ChrisJ

Hi Chris...AA frame and engine numbers didn't match from the factory...I can't verify the story about 'matching up' replacement engines but either the AA or someone else has done it! The basic bikes were the same as 'civvy' versions apart from AA specific parts which are listed in the parts lists for each year. The petrol tanks are as WD versions apart from the fact they have a smaller filler cap and it opens sideways. AA specific parts included the retention of the single spring clutch I believe, so that may be a clue if the owner knows what's on it.
For the last 3 years of production (60-63) they were only supplied to the AA and these late types had an alternator fitted as well as a dynamo. These engines are prefixed 'BM21A'.
As a point of interest the M21 was the last production sidevalve vehicle in Britain...Ian

Thanks Ian. A sideways opening petrol cap would be a good clue to the bike's provenance. Unless of course it's a real bitza! I'll go and see it and figure out what to do from there.
ChrisJ