Anybody know whats up with my pistons?

[pbsun]

Doing an engine rebuild on an m20 that I just bought and found dings on the pistons, anybody have experience with this? If so do you know the cause? Also what did you end up doing; keep them or get new ones?

Edit: These dings occur on 5 of the 6 pistons in more or less the same spot on each one.

Thanks.

[LJ851]

Looks like some debris got into the combustion chamber(s) and dented the piston. Are there corresponding marks on the head?

I would file the high spots off and smooth any remaining sharp edges, they will be fine to reuse if otherwise in spec.

[EyExR]

detonation?

M20, okay. But what type of setup is this from?

[ak-]

detonation?

I think detonation would have scattered dimples all over the top of the piston rather.

[whodwho]

Looks like some debris got into the combustion chamber(s) and dented the piston. Are there corresponding marks on the head?

I would file the high spots off and smooth any remaining sharp edges, they will be fine to reuse if otherwise in spec.

This, and look for any embedded metal that could dislodge and continue the journey.

I think detonation would have scattered dimples all over the top of the piston rather.

Agree with this as well, would look like a really bad sand blast job 8^)

[pbsun]

EyExR this is from a stock m20b25.

Thanks LJ851 and whodwho my fear was that I'd have to replace them but it sounds like they will be fine if I shave down the rough area. So the remaining question is what could get into each of them to do damage to this specific section of the piston?

Here are pics of the head, looks like it has dings on it as well.

[Cabriolet]

this is very very common. out of the 10 m20s I've rebuilt 9 of them have had this. it's from carbon building up and then banging around. dont worry about it at all.

[pbsun]

So your saying it would have been a thick layer of carbon build up causing the piston to hit the head?

Just to make sure I've covered all bases here, any reason this could have caused issues in other parts of the engine?

BTW, thanks for all the replies.

[MR 325]

You could smooth out those spots and run them without issue.

Or you could always buy another set of used pistons. They're pretty cheap usually.

[z122643]

You've got the engine apart, I'd do a light hone and new pistons.