My brother in law brought me his 350 out of his 78 z28 to rebuild for him. I was tearing down the engine today when I noticed the #4 pistons compression ring was broken. No big deal, I thought. However, upon further inspection, I found that both bottom rings were being pinched by the piston- that is, while the top of the piston was flat, from the compression ring down in one small area was literally pinching the piston rings, preventing them from moving at all. the piston was actually sagging in that one area. however, the top was flat, the piston is round, and the cylinder is unscathed.I have NEVER seen anything like this. can someone explain this to me? If need be, I can post pics


and the car has .030 forged popup speed pros........

Borderline piston to head clearance maybe?

Post some pics.

Ill try to post some tmrw- def not valve related though...........

Is he hitting the juice on this engine? Or it could be some bad detonation.

Sounds like it was ran hot quite a few times. How much of a ridge is there at the top of the cylinder?

In supercharged engines I've seen where detonation causes the ringlands to fail BELOW the top compression ring. Sort of just pushes them down. I suppose it could happen with nitrous, too.

Edit: I should add that it has a kind of "melted" look to it, not just mechanical collapse when this happens. I will see if I can find a picture of it. If it's just mechanically compressed it's likely something just got jammed between the pison and the head while the motor was spinning (mechanical carnage).

It is definitely the most bizarre looking failure and I was not able to diagnose it when I first saw it. I eventually ended up calling KB's chief engineer (he actually got on the phone with me!) and he explained it to me. I doubt I could do justice to the explanation, but the upshot of this conversation was that many KB pistons have a deep V-groove between the compression rings to help prevent this exact type of failure.

it may have had nitrious in the past, but not since he has had it.



no ridge in the cylinder.


no supercharger, either.


the pics I took stink- will try to get better ones to post...........