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Hi Chaps, I am interested in BSA M20 cams. I have some Phil Pearson clubman cams. I was wondering about putting them in my bsa m21. Any advice on this and altering valve timing to suit would be very much appreciated. Also I am interested if anyone has one an alloy cylinder head they wish to sell. Thanks Chaps Long live the M20! Harry

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The BSA sidevalves are basically stationary engines that were fitted with wheels...In other words they are in an extremely low state of tune with everything about the design making them an understressed, low power unit that was primarily intended to be both simple and durable...In that the aim was achieved...
One interesting point is that the M21 was the last production sidevalve for road use built in the UK...Due to the overall design and its inherent limitations there is no 'quick fix' that will give a boost in power...Cams, for example, have to work in tandem with other features of the engine to achieve gains...Put in a big cam and things are still limited by poor porting, overly heavy flywheels, a poor cylinder head design (for gas flow), low compression etc. etc....If it were not so the Gold Star would be a sidevalve with a big cam....The quickest gain with the least research and work can come from enlargement...My 720cc 'M20' does go better but not as much as you might imagine from a 40% capacity increase..The other limits still apply...
Looking at it another way consider the B33...500cc, a relatively low compression ratio, fairly heavy flywheels and the same cams as an M21...However, it has the benefit of an OHV design with all the benefits of that type...The result?....23 BHP, an increase of 10BHP or approx. 80% over the M20...
So in conclusion I wouldn't bother with anything other than standard cams unless you also do some carb, porting and cylinder head work (at least) and DBD34 cams (if that's what you have) wouldn't be the best choice anyway without extreme re engineering of the engine specs....Ian

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