My M21 has oversize piston rings of 0.080" of an inch. This I think is the limit. Does anyone know what I should do after this? Is it still possible to rebore and what size piston and rings should I fit afterwards?

I have a notion that upgrading the carboretter on my M21 to concentric from the 376 mono block may be a good step. I figured it would fit closer to the inlet due to the float chamber being underneath, this the dynamo went get in the way and I am informed the bike should run more evenly. Could anyone please recommend a suitable concentric carb for the M21?

I noticed while decoking my M21 that there was one broken piston ring so I needed to replace it. Interestingly the piston is +0.080. I have never heard of this before and thought +0.060 was
as big as it got. The bike is registered 1958 but the engine is 1954 so this would be a cast iron barrel? Certainly the weight of it confirms this. Does anyone know if cast iron barrels are known for being bored out to this extent? The barrel bore is beautifully smooth and piston clean so I assume the re-bore was recent but unfortunately I got very little history with it. There were one or two other little problems, now more or less sorted, which would explain the price I paid - £4250. Lovely reliable bike though. Just come back from touring the west country with the missis. We had a grand time.

I wonder if it is possible to alter the gearing slightly on my M21 by put a sprocket on the back wheel with less teeth. Would this give the bike a bit more speed? I don't want much more it's just that I can move off happily in 2 nd gear as 1st is for climbing trees and as the bike is solo is more or less redundant.

Oh dear, getting confused again, maybe it's old age but I tried to fit a NGKB5ES spark plug into the 1958 M21cylinder head but noticed the plug I took out was shorter. According to the info I had, the alloy heads have shorter plugs - 1/2" and the iron have 3/4". So I wondered if I had bought the wrong one and after putting it in, very carefully moved the piston up and see if it was obstructed. Sure enough it was and on taking it out noticed that the gap had been pressed closed and a smear of carbon was on the top of the plug. I thought the alloy heads required the long reach plugs not the shorter ones. This being the case, is it possible that my M21, 1958 has an M20 or older M21 iron cylinder head on it? Is this possible? There is considerable rust on the fins which would flag up iron as a possibility but I thought by 1958 all these heads were alloy. I checked the other parts I have already bought, valves, springs and guides and they are universal to both M20 and M21 so I have no worries there. The other potential problem is when i come to doing the piston and rings. The M20 appears to have 4 (I believe one of the groves is not used), and the M21 only 2 rings. The stamp on the engine onto where the barrel sits states plainly M21 so should I expect a 2-ringed piston, or indeed if the barrel is an M20 could I be further confounded by finding 4 groves! I have always believed people when they said the M range are simplicity itself but I am now wondering if it's me who is simple. Please can one of you chaps who know these things explain what could be going on?

The contact breaker assembly on my M21 revolves when i use the kick starter. I am sure i have never seen a contact breaker assembly revolve like this before. I assume this is why i am not getting a spark at the plug because the contacts are not opening. Would this seem a resonable diagnosis? Please could someone who knows help me out? How can I fix this problem.

on my M21, which is running nicely, i recently found that after revving up as the revs go down there is like a couple of bangs in the silencer. This is a new symptom and it's getting worse. Sounds like the baffles have started to disintegrate and are no longer silencing effectively. Maybe making exhaust gases not operating under the correct pressure. Would this sound about right? It does seem a bit far fetched that baffles would just break but everything was running so well, lovely tick over, mixture set correctly and no other mis-firing. I recently cleaned the carb and jets plus contact breaker and spark plug. Any suggestions would be welcome please.

The old M21 engine is running beautifully but a new noise on the bike has occured which is causing me concern. Trouble is, it is difficult to hear where it is actually coming from. When decelerating I notice a grating rattling kind of noise like a metal part vibrating against another metal part, then it stops as I slow down. Seems to happen around 25 to 30mph when reducing speed and not on accelerating. Could this be something like a slackened chain which is too loose? I know that all parts have their own particular resonance which make them vibrate at certain frequencies. Would a drive chain rattle against another part when decelerating? It has about an inch of movement when pressed. Could there be any other parts which can cause this symptom? What really worries me is that it might be problems inside the gearbox which is far beyond my technical capacity to remedy.