hey H-T,

recently had to put a new piston in my motor (melt down) and finished it up and everything was running fine. drove it for 200 miles, blew my oil pan gasket, spun 3 rod bearings so i pulled it apart to re-bearing it. i was inspecting the pistons and 1-3 were flawless and had no problems. then i get to piston 4, the one i JUST replaced and re-reing the whole nine yards, and there is a hairline fracture on the ring-land again! i clean it up for a closer look, start to take the rings off and "pop" a chunk of the ring-land is in my hand. WTF?? i have no clue what could be causing this to happen on just this one cylinder and not all of them... and in the exact same spot none the less. (injector/intake side) so at this point im going to take it to a machine shop and have it looked at for some opinions, but i would also like some educated opinions off H-T as well to see if anyone else has seen this before. I am thinking it has something to do with running lean, but then again it would melt the top of the piston and not break the ring-land.

pictures are below, the new piston is on the rod, the old one is the one thats melted. im running a ls built GSR on 10 pounds of boost as my setup. once again thanks in advance for input.


piston w/ missing land by its side


comparison... exact same spot and problem


close up

 

is the motor tuned?

 

yea. it was dyno tuned on crome pro a few months ago.

 

BUMP

 

Sounds like you put a piston and new rings without removing the ridge at the top of the cylinder. So the new rings were hitting the ridge stressing the piston ring land and breaking it.

 

X2
or just bad pistons...

 

im not quite sure what ridge you are talking about. there should be no modifications needed to be made to a picton before ringing it and breaking it in. i re-gapped the rings and inspected the piston before hand

 

Is the cylinder true? If one side of the wall is flattened then that could be rubbing the piston and creating too much heat. If there is a ridge I think you would have seen that during the re-build. Maybe not. Check it. Also are all of the injectors working fine. Leaning out is usually the reason for melt-down. Also is it a reputable tuner?

 

at the top of your cylinder wall there will be a lip from wear of the piston, your rings dont ride all the up the cylinder wall leaving a ring. You need to used a cylinder hone and take that lip off before installing new pistons. it is a very simple process and you can tell where it is because it is generally a darker color.

 

I was always under the impression that if a lower end was going to be redone the crank should be balanced and taken to a machine shop to be tested. That said, at this point you might want to take your block out, maybe the bore isn't perfectly circular anymore.