Hits in piston and head
 

Hi Guys,

today we opened my engine and everything looked great until we got to piston two. It seems that it ran too hot for a while for whatever reason and had some knocking.

https://www.miataturbo.net/attachmen...ine=1393354204

https://www.miataturbo.net/attachmen...ine=1393354204

Question is, can we use that piston again? My buddy got the opinion if we sand it down it will good for another run.

What do the experts think? Thanks for sharing your thoughts!

your buddy is an idiot

no you cannot

Looks like it could be salvaged for broke ass build, how much is a new one?

What can happen if we use it again? Didnt check the price of a replace yet, but its not a broke ass build and supposed to run on tracks.

For the record, my buddy got some good reputation as mechanic and does it also for living. But I am suspicious as well thats why I am gathering more opinions.

If you sand it down, you'll need to JB weld some quarters on the domes to get the compression back to the same level as the other cylinders. Use a nickel on top of the piston if you take more than 1MM off the top.

If it were me and I had the thing apart, I would get a set of replacement slugs. And I am incredibly stingy.

I totally would keep using it.

Yeah I dont see the problem. Sand the sharp edges down and put it back in. Except that the engine's probably going to need the next overbore to get the correct cylinder wall surface finish for new rings to seal.

Why did you pull the engine apart?

The cylinder walls and piston rings are totally fine. It got a bore job in 2006 and it still looks like new. The wall of the piston looks good as well. Why do you want to overbore it?

I took it apart to to switch to H-beam rods and ARP bolts plus a general overhaul after some years ob track abuse.

This thread is really going to help me with my decision. :rofl:

According to the official woessner price list a single piston starts at 310$. I will call them tomorrow and see whats their phone price will be...

That's not knock. That's foreign object damage. The piston is not reusable. The head can be surfaced and fixed.

So also check your throttle body butterfly valve to see if you're missing one of the screws. Probably not, but seeing that, it's worth verifying.

no. those brass screws barely leave damage, and the imprints looks just like a screw...

his looks worse, but aint nothing but a red badge of courage.

I've run pistons worse than that, but not for track use. It was an old engine, and the broke owner was not about to spring for new pistons. Ran fine for a couple of years afterwards until the frame rusted out. Was an S-10.

It needs to be stoned down to remove the edges around the pits completely.

Those brass screws destroy motors.

Just turn them into singh grooves.

Meanwhile I totally agree that it is not knock but some foreign object. I turned and mirrored the pic of the pistons and the hits totally fit to each other.

https://www.miataturbo.net/attachmen...ine=1393449528
https://www.miataturbo.net/attachmen...ine=1393449528

Still not sure about replacing or not. Woessner doesnt know if they have a replacement, I am waiting for a call after they turned their storage upside down...

Thanks for sharing your opinions. I will keep you posted!

I disagree. I think that looks exactly like bad detonation, exactly where you would expect to see it.
If that was a cast piston it would have cracked/melted.

Detonation that bad is not localized like that. It's very obviously foreign object damage.

Not true. On combustion chambers with quench pads that is where you normally see the pitting start.
It looks to me like it experienced some hard detonation but not for very long. Op was obviously observant enough to catch the problem and not keep beating on it.

Higher resolution might help.

That is not damage from severe knocking. Clearly the motor ingested an object that caused the damage.

On another note, a pet peeve of mine: There is no such thing as 'detonation' in an engine. There is knocking combustion, which is a result of autoignition of the air fuel mixture in a region of the combustion chamber prior to the flame front consuming it.....

Detonation:
https://www.miataturbo.net/attachmen...ine=1393456600

Knock:
https://www.miataturbo.net/attachmen...ine=1393456600