Piston Autopsy

I just measured the minimum distance between the piston pockets and the back of the top ring groove in an un-modified 83 mm piston, as delivered from JE. For the intake pocket it is .215", and for the exhaust pocket it is .175". The accuracy of the measurement is probably within .002" either way. That should be more than enough safety margin to prevent melting through under normal race conditions. Under severe detonation, or prolonged lean running, or too much advance, or all the above, maybe not. I don't really have any significant experience with those conditions.

Ken

Ken

Yes .215" is plenty of material but the piston in the photo looks to have less than 1/2 that much material when measured vertically from the pocket to the ring groove. The combined horizontal measurement must have made it .215" (unless the pocket was re-cut which I doubt because Maney's pistons fit his big valve heads).

So there was some serious overheating and/or detonation going on to cause that much damage when there was enough thickness in the piston deck.

jseng1 said:

Ken

Yes .215" is plenty of material but the piston in the photo looks to have less than 1/2 that much material when measured vertically from the pocket to the ring groove. The combined horizontal measurement must have made it .215" (unless the pocket was re-cut which I doubt because Maney's pistons fit his big valve heads).

So there was some serious overheating and/or detonation going on to cause that much damage when there was enough thickness in the piston deck.

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Yes, the vertical spacing is .124" and the horizontal spacing is .176", which gives an angle distance of .215".

Ken

I just communicated with the owner of that piston and he says the vertical dist is 2mm and the actual material thickness when measured on an angle is 3mm. So it sounds like the piston was modified and the thin spot could have been the problem if there was overheating and/or detonation.

I have seen pistons survive with less than 3mm material thickness at that spot as long as they didn't have overheating problems.

All just looks familiar to me

Detonation. Too much timing advance or not enough octane.
Lack of seizure marks means the piston was not seriously overheated before the ring lands were broken.

PS, JE rings are steel and can be bent without breaking -unlike a cast iron ring.

Chris said:

All just looks familiar to meView attachment 10900

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Yours looks like not enough clearance for the piston temperature. The seizure would have happened before the ring land failure.

It's tough to get a skirt seizure after the ring lands break since once they break there is no more combustion heat to cause expansion.

Of course the piston may have been overheated due to minor detonation that was not severe enough to cause a piston to break until after a seizure severely overheated it.

Hi Comnoz

You & Dances identified the cause of my destruction. Big bore far to much advance & my fuel not being rocket fuel.
Lower compression & 28 degrees advance have sorted that. Well that & a new barrel & pistons.

jseng1 said:

I just communicated with the owner of that piston and he says the vertical dist is 2mm and the actual material thickness when measured on an angle is 3mm. So it sounds like the piston was modified and the thin spot could have been the problem if there was overheating and/or detonation.

I have seen pistons survive with less than 3mm material thickness at that spot as long as they didn't have overheating problems.

Click to expand...

Yes. After looking at the picture more closely, it is clear that the valve pocket has been cut deeper and closer to the edge than original. I think it would still have been OK with less material thickness in normal race use. Detonation does seem to be the most likely explanation.

Ken