Cause of cracked oil ring land?

[medarwin]

Hi,

This is what I found when re-assembling my engine after head and manifold stud replacement. There is a crescent shaped crack and the land is pushed up towards the top of the piston. Sorry for the crappy pic, best I could get that close.

Anyone see this before or have an idea of how it happened? 1982 911 SC, 95K miles. SOmeone has been in the engine before me, don't know when or why, I have owned the car for the last 28 years. Any insight appreciated. Thx, Darwin

[BURN-BROS]

I cant see anything...Which land?

[HaroldMHedge]

This has been discussed in the Excellence Technical Notes.

Originally Posted by 303462

Here are a couple photos of the Technotes article. Sorry for the poor photos, and you will have to go back and forth a little to read it all. Could not get a full page in and have it legible. Also a couple shots of my 83 row parts. I never heard of this til I experienced it.?

Taken from: Anyone driving an SC 930-10 row?

[medarwin]

THe oil scrape land above the wrist pin. The top edge of the bore for the wrist pin is cracked and pushed up the bottom of the land.

Detonation makes sense. I never had an issue with it but I guess that kind of stress could do it.

[jjeffries]

MEDarwin, does reading that article give you any pause about just replacing the one bad piston? John.

[BURN-BROS]

That could be an install issue.
Did you have any broken top rings?

[pmax]

Medarwin,

Correctly me if I read it wrong, it is amazing that the engine was supposedly running well (!) with this.

Any idea when the damage may have occurred in that 28 years ?

[medarwin]

Running just fine. And with 5 broken head studs. No reason why it wouldn't. The disfigurement did not cause the piston to contact the cylinder wall and as I said, the oil ring moved freely and was not broken. The only danger was that if the crack worsened and the piece broke off and ended up in the crankcase.

As I said, I beleive it was caused by wrist pin installation at some point. This is the first time I have had it open in my ownership. It was 8 years old when I bought it....

[medarwin]

No broken rings. Oddly, the rings that came out moved freely in the constricted land. The damage was discovered when the new ring couldn't be seated into the ring groove. All other pistons inspected, no similar situations. I have to believe that the engine went quite a few miles in this condition. ANd it might have gone a lot more, no way to tell.
I have to say that on further thought, I believe this was done when the wrist pin was installed, either in the factory or by someone who had opened up the engine before I had it. Detonation can't be the issue because the wrist pin isn't touching the piston there.

[76FJ55]

Have you tried installing the original rings, to verify that they actually moved freely? Could it have been damages when the pin was removed during engine teardown? A better pic may help diagnosis as there is no evidence viable in the original post.

[Dodge Man]

Originally Posted by HaroldMHedge

This has been discussed in the Excellence Technical Notes.



Taken from: Anyone driving an SC 930-10 row?

Harold's link is worth a look. My 1983 930-10 motor seemed to be running below par. Motor was from a gray market car. PSK checked my cam timing. My cam timing was set to "US cam timing specs" to pass SMOG when imported. PSK reset my cams to Euro spec and my 930-10 runs "happy" now.

What does running a 930-10 at US spec cam timing do to cylinder pressures?

What would running 35 degrees BTDC instead of 25 degrees BTDC add to that?

Just asking if cam & ignition timing could be a factor in low mileage failures?