Cracked Ring?

[nik97]

Ok, long story short guys and gals. The completely rebuilt 3.2 in my car started to develop a pulse from the atmospheric crankcase vent along with a slightly more than usual vapor flow. The pulse is the same frequency as a single cylinder at idle indicating some excessive blow by in that cylinder. Did a compression check which yielded the following: 1,2,3,4,6 195-205 and # 5 at around 175. Next I did a leak down test to reveal the source. #5 showed moderate(50%) leak down while all others showed low(10-20%) and the shop air was escaping through the crankcase vent.

This motor has about 3,000 miles(2K n/a break in) on it and has not seen more than 4 psi loaded. The AFR drops to 10.0-11.5 on boost and EGTs stay in a safe range. The IAT is the only thing pulling timing and the motor does not ping or detonate. Knowing all of this, I was still very worried and I tore into it right away. Pulling the heads revealed pristine cylinder walls and pistons/chambers save for the evidence of a healthy fire burning within.

So what could be causing this blow by all of a sudden? I'm no expert but it seems to me that a cracked ring would still only leave a sum gap equaling that of the original ring gap... Right? Also, I would sort of expect a cracked/broken ring to damage a cylinder wall causing some blow by but I did stop driving it and right away. Another thing I considered is that if the ring did crack, is it possible that the crack is directly above the second ring's gap causing lower than normal compression? That sounds a little unlikely but at this point, there's not a whole lot else.

I'm going to remove the piston/rod tomorrow but I thought I'd get some input from those of you who have seen it all. Any input is appreciated. :wave:

BTW/ they are Hastings rings.

Thanks guys!

 

[nik97]

UPDATE:

...Well, not really. I pulled #5 piston/rod and found nothing. Piston looks good, rings are good. I placed a new ring in #5 bore and another to compare end gap and measured ~.001" variation which means nothing.

I'm confused.

 

[93rev2sev]

did you check the piston to bore clearance or did you trust the machine shop?

 

[nik97]

After a closer inspection of the piston with more light and less alcohol, I found a cracked ring land. There where two small cracks about 2 inches apart and a little tap with a screwdriver loosened t\a large chunk of piston. Fortunately, the cracked piston stayed together and did not damage the cylinder wall at all. I'm replacing the bad piston right now and maybe I'll post some pics here in a few.

Very relieved to find the source of the problem and be able to repair it with minimal cost.

 

[Adumb]

i think nik97's real name is gregory house

good job!

 

[yamahaSHO]

I was just about to question the ring land. Pat has broken three of them now on his stock boosted motor. They don't damage the cylinder wall, but they take some work to replace. If you're on stock fuel and/or stock fuel setup, you'll see them break on cylinder 4 and 5 regularly without proper charge cooling and timing control.

I switched to Josh's high performance fuel setup to avoid this. It feeds both fuel rails independently.