blackeg

my last lsvtec met its end from some scoring on the cylinder walls at the top of the bore.

one of the thoughts on the cause was piston expansion. ptw was set at .0015 and they were 81.5mm rs machines pr3 pistons


piston skirt


cylinder scoring

so i honed the scoring out, but opened up the ptw in the process. whats the highest ptw you guys would use on a cast aluminum piston before you run into problems like blow by or excessive wear

c5R

Judging by the condition of the piston you did not have a P2W problem piston still looks food looks like the rings were filed improperly and caused the damage.

Charlie Moua

iTrader: (3)

hum....
is it like this in all cylinders?

it depends on how acurate the measurements really were because .0015 seem plenty fine for cast alum piston.

having more p2w doesn't nesscarily mean you will have more blow by. The ring gappings would dictate blow by more so than p2w clearance (within reason lol).

blackeg

elaborate on improper filing. i did these the same way as the other motors ive built and not had this issue before. whats the best recommended way to file

most cylinders had a bit of wear, but #1 cyl had a scratch that broke the crosshatch and caused a drop in compression from 240 in the other cyls to 200 in that one.


the other theory that i got about the scratches was carbon got caught between the piston and bore and scratched it up. seem more probable than incorrect ptw?

GhostAccord

I can tell you that it was more than likely not carbon that did that. I have taken apart my share of carbon coated engines that have never had scoring like that. Those scratches look as though they are from metal debris or sharp edges/a bur on the ring ends.

blackeg

ok, carbon ruled out.

funny part is that i dont have any of my ring end gaps there.

since its toward the top ring would it be metal or other debris getting into the cyl through the valves, instead of from the oil which would come up from underneath and be more on the skirt or further down in the bore ?
i appreciate the help and feedback. i might be reassembling this motor again this year and dont want a repeat performance/trying to learn from my mistakes

GhostAccord

Rings will rotate in the cylinder as they are being broken in. Usually once they have settled in a path they will settle and not move unless disturbed. Considering that the scratches end/start exactly where the top ring stops. I would suspect the top ring as a possible culprit. If it is not the ring then it is quite possible that debris did come in from the intake side.

blackeg

good info there. usually between filing the ring and putting it in the bore to check the gap i hit the edges of the side i file with very fine grit sandpaper for a split second to debur basically..

i also changed manifolds between portmatched p30 and sk2 pro series a few times once the motor was installed. i tried to be as clean as possible but i wonder if there was something in the intake manifold that got loose and into the cylinder. the compression loss occurred with about 13,000 miles on the motor, and probably 5,000 miles after i put the pro series im back on for good.


just for *****, would light detonation cause the scoring, especially before it was visible on the piston with shiny pits