I am replacing the top end on my M21 due to what turned out to be a broken ring. A couple of questions, On it's previous rebore I was supplied an over sized gudgeon pin with the piston kit and I reamed the new bush to suit. On dismantling the top end now, that bush is black and looks burnt, I'm wondering if I reamed it too tight and has over heated? from memory the pin was a good push fit in the bush and the piston would flop to it's side when let go.



Secondly the new gudgeon bush doesn't have an oil scroll machined into it,(it does have the oil hole drilled in the top though) should I get this scroll put in?

I think its best to have the oil scroll, though machining it can be tricky..

An alternative is to file a small groove along the length of the bush to intersect the oil hole...You can also make the hole into a slot to conform to the slot in the con rod...

The fit of the pin is difficult to explain but I have definitely found a tendency for the small end to overheat if the bush is too 'snug'...The pin will sometimes become discoloured as well due to heat build up...

So, ream the bush so that the piston falls easily to each side under its own weight with no resistance to movement...But take care not to overdo it..Ian

email (option): ian@wright52.plus.com

Thanks Ian,
There's some discolouration on the pin where the ends of the bush sit but more telling is the indication that the piston has been rocking on the pin and not the pin in the bush.



I think taking care not to over do it last time was an issue