dave0415

Has anyone else out there had problems with cracked pistons? I was in the middle of reassembling my 96 12 valve after having to surface the deck, and repair a crack. I had replaced the pistons less than 30k miles previously and was very surprised >to see 1/4" to 1/2" cracks around the edge of the inset in the piston. The original set of pistons went 400k miles and were replaced due to ring groove wear.

Don M

Where exactly were they cracked? Also, can you describe your block preparation for the new pistons? Piston to wall clearance, etc. Ring gap?

Don~

dave0415

Don,

They are all standard size, and were within tolerance when installed. I still had OEM hone marks on the cylinder walls when the new pistons were installed. The attached picture shows where the cracks are they are in the same area on all the pistons. The crack is the dark line by the piston numbers. The picture blurred up quite a bit in the process of getting it small enough to fit. One of these days I'll figure a way to beat that problem.

dave0415

After many offline discussions, I have concluded that I have been bit by too much advance in the pump timing. Causing too much cylinder pressure/heat, and the gasket/piston problems.

MCummings

Where was your timing at?

Merrick

dave0415

We did not double check it when I pulled the motor apart. Rough guess from how far I had to move the engine to get to TDC after locking/removing the injector pump is 21-25 degrees.

Jim Fulmer

For those of you that don't know what 25 degrees sounds like, if you have your head under the hood while it's idling and rap the throttle it will hurt your teeth. Dave I'm guessing that you have a 5 speed, if you do and your timing was at 25 then it would have been about 1 1/2 inches or more back from pump TDC.

Jim

98rammer

21-25*? yikes. still, would that create enough heat to crack pistons? general cruising is only 600-900 degrees usually. hot spots or something? i'm fearing looking at my pistons after hitting 1850* a couple times

Tom

dave0415

98 rammer,

I did not see high temps with this problem. What I got was too much cylinder pressure which blew the head gasket. Which let go between # 5 cylinder and a coolant hole letting combustion gasses turn the anitfreeze into a green tinted milk froth. :'( Unfortunately for me I was under load and 1200 miles from home when it let go> I ended up having to pour antifreeze through the engine and limp it home. By that point I had deck/head warp issues to deal with. Fixed those and found the cracked pistons when I started reassembling everything.

Jim,

Yes I have the five speed. I'd estimate 1 1/4 inches to TDC

Dave