About 3k miles ago I did some Prc 13 degree 227s, hollow valve's, milled to 59cc, tsunami cam, flyout, 1 7/8 headers, ported fast 92 and all the bolt ons. Timing is 28* degrees and no signs of knock when tuning. Afr was around 12.1-12.4 wot.
Bore camera shows no sign of cracks on any pistons but # 7 failed a leak down and only 90psi.
Thinking about putting another stock piston in and throwing it back together. I just don't want another issue. Anyone have an answer why this may have happened?
My c5z has been running a similar setup since 2004 with no issues but AFRs and g5x3 cam.

 

Well that sucks that #7 was 90# and failed leak down.

Had the same thing happen with a junkyard LS1, I bought a long time ago. Likewise had a 90# leak down. We pulled heads and found, it already had a stock replacement piston in the #7 hole So yes putting a stock replacement in can work sometimes. However, by the time the LS1 got to me it needed to go .005 over to get a good seal.


Noticed that Afr was around 12.1-12.4 wot. Is thatlittle on the rich side?

Most NA LS seem to be in the 12.5 to 13.1 range. I'm not a tuner so hopefully the experts at Tuning will comment.

FWIW no answer on #7 but here's a theory - The way it was once explained to me: #7 may see a little less airflow but gets the same amount of fuel so sometimes #7 is richer. That may cause #7 to run a hair hotter than the other cylinders because it's tad richer than the average cylinder. Same amount of fuel, but less air, so richer. Richer = hotter as a result

 

Low cylinder pressure could also be caused by bad sealing of the valves. A bent stem, burned valve or worn seats could give similar results. Are you able to hear where the air is leaking to during your testing?

 

No air coming back threw the intake. Motor has 24k miles and 2k on the Head and Cam. 12.4 is alittle rich but the car ran good and pulled like a freight train so we left it alone. I was thinking maybe a touch too much timing with the 13 degree heads being more efficient? I will drop the heads off to get the valves checked when the motor comes out.

 

I would agree with this assessment, while 27-28 can be fairly normal on a factory headed Ls1, moving to a more efficient head usually reduces timing as its more efficient.