M21 wet sumping over winter. Tried to start it and got utterly jiggered. Put on new rings, new carb, emptied the sump, new spark plug, new hand made copper, (2mm)- not composite- gasket and brmmmm second kick. Tuned up the carb, new gasket on the valve gear cover, which needed tweaking as did the cylinder head and now starts first kick. Running like a good 'un. Thanks for all the help in the owners club.

I took the head off my M21 and is has loads of oil in it plus the carb is blackened. Does this mean the piston rings have failed? I put +0.080" rings in but still have the same problem. Should I fit a larger piston, if so which is best please?

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 got one of those gadgets to put on the oil pipe to stop wet sumping. Which pipe does it go on the outer or inner?

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 may have need of doing a journey starting in Oxford going to Reading at 0400 in the morning by M21. It's going to be, maybe
minus 8. If I were to lower the pressure on my tyres, would that give better grip?

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 anyone else has a fascination with setting the tick over on a warmed M20/21, maybe retard a bit and then see how low revs they can get
the tickover? Is this just me just interested or has anyone else tried this?

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.

Just about to put back my M21 barrel after having hardened valve seats put in and one of the piston rings "came apart in my hands". I don't think it was me as I did not try taking it off but at any rate I need a new set or at least a new top one. Scouting around and reading about them it seems that getting +060 rings is not so easy as I expected. Do people usually have these made when in this situation? I wonder if rings are more or less generic in that provided they are the correct size and type it doesn't matter what is used. I was also bewildered just how many different metals are used for different types. I wonder also if I could fit +040 or even +020 if I made sure the gap in the ring is big enough?

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?

My old dad had a double adult sidecar on his Triumph 5T and as a toddler when trundling around Sheffield, I remember feeling it was warmer outside than inside the sidecar. Later on when I was old enough to sit on the bike "helping" my dad with the regular maintenance I asked him about how to get the sidecar heated and he went into elaborate detail on his plans to redirect the left-hand exhaust pipe so it ran through the side car and then out the back so as not to poison the inhabitants. He never did get round to trying out his invention and I have been left wondering if it was possible and if so, why has no one else done this? Or did my dad miss the chance to get a patent on sidecar heaters and thus make a mint? I wonder if the idea is feasible because I may well try this on M21 when i get a sidecar for it. One initial thought was that having 2 different lengths of exhaust pipe may interfere with the balance of the exhaust gas pressures. On a single this would not pose a problem. My old lady said there's no way she will ride in a side-car without serious comforts so I am on a mission.

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.

Hi, I am about to take off the cylinder head of my M21 to de-coke it. At the same time I plan to re-grind or replace the vavles and springs along with a new head gasket. Is it necessary to put on new piston rings at the same time or will I be able to get away with leaving on the current ones? By not replacing the rings saves me having to take off the barrel which on the M21 is bolted to the crank case seperately from the head which itself has a further 10 nuts. It appears to be possible to grind or replace the valves also without taking off the barrel so I could achieve quite a bit without taking much apart. One niggling issue for me would be that since the new valves will create a better seal, thus higher compression, would this cause problems with rings that are a bit worn?

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.

I have a side stand on my 1958 M21 which is obviously not the original, (if there ever was one), but no matter how much I tighten it up when I use it the weight of the bike makes the stand slide round the frame and the bike falls over. Is there a cure for this or is it that the M21's are just too heavy for side stands?

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.

bike starts and runs nicely now thanks to advice from you chaps ironing out some little bits and bobs. It has just developed a bit of a leak where the exhaust pipe goes into the cylinder. I can both hear it and feel it  and although the leak is not major I assume it might get worse so since there is no tightening ring which i was used to on non-British bikes that I rode before returning to motorcycling in my later years so how do i tighten up the join? There are no nuts and bolts on the outside at all so maybe it has to be done from the inside?

After solving one problem with a 376 carb on my M21 a new one has developed. Every so often the bike will not rev up and i found the needle valve was sticking in the closed position in the float chamber. Why would this suddenly happen? Does anyone know of a cure?

My M21 was running ok but i would like to slowly go through the bike and sort out problems so i put new jets, needle, float and gaskets in the carb and now although it starts easy enough it runs for a few seconds and conks out. All the settings are as they were previously, the jet sizes are the same, so i just cannot understand it. The spark plug is sotty and yet weakening the mixture makes no difference. Maybe i should just put it back as it was before but now i am worried it still won't run properly. It ran fine before, just a few little ups and downs but i thought no harm can come of replacing old bits with new ones which are exactly the same. Would someone take pity on me please and give me a suggestion or two?