My M20 had started to back fire on the over run, so I decided to have a look.

Any ideas as to why this has occurred (see photos).

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Hope some pictures are shown.

Regards

Pat

email (option): sacombsashtrees@hotmail.com

Forgot to mention that the damage is valve side and the liner at the top doesn't look great either.

Circlips all present and undamaged.

Regards

Pat

email (option): sacombsashtrees@hotmail.com

Hopefully I have posted some better pictures below.

Regards

Pat


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email (option): sacombsashtrees@hotmail.com

Looks like a non standard or pattern piston. I think you've caught it just in time. These things happen. Try again with an original piston.

Hi Pat..Could you post a picture of the top of the liner as well?...Ian

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

It is a Hepolite piston.

Having spent a bit of time on the liner, the liner doesn't seem damaged. Just a lot of deposit build up on the bit where the piston has been damaged and only at TDC. This will polish out. There is a tiny lip at the top (all round), but nothing (I think) to worry about i.e. I don't think it is re-bore territory.

The liner isn't scored in any way. I had thought that it had dug a ridge in the liner at the top where the piston had been damaged.

The piston itself doesn't show signs of scoring, though there is some tiny evidence of wear front and back.

The gudgeon pin came out a bit easily for my liking i.e I popped it out with my finger without warming or drifting, though no discernable wear or play where it enters the piston.

Small end looks and feels ok. There was no piston slap before.

The only reason I started down the strip down was that it was popping on over run i.e. I thought the exhaust valve or guide needed some work.

email (option): sacombsashtrees@hotmail.com

I think that the damage could be down to pinking.

email (option): sacombsashtrees@hotmail.com

Hi, this happens with original piston too. I think too much carbon causes this damage. I try to inspect and decarbonize every 1000 miles or so.

I have quite a few pistons in my shed like that. Carbon build up where the exhaust gasses scrapes on the edge of the cylinder line before it leaves though the exhaust valve.

The carbon build up is stronger then the piston.

Henk

email (option): ahum@quicknet.nl