What causes a cylinder base seal to split?
 

I know that these little copper shims are fragile, but what could have caused this cylinder base gasket on my 3.0 to split like this?

Poor installation, overheating, incorrect deck height? This gasket split on #6 cylinder, causing a significant oil leak. No head studs were broken, and the heads were not loose at all. No imperfections are visible on the block or cylinder base faces. The engine has 240000 km (150k miles), and was supposedly rebuilt once before in its life. The full history of the previous rebuild is unclear, however, but I guess these are standard items to replace when the cylinders have been removed (of which I have a photo record from the previous owner).

http://forums.pelicanparts.com/uploa...1108707526.jpg

Upon disassembly of the heads, this broken valve stem seal came out in two pieces. I'm also curious why this would have broken?

http://forums.pelicanparts.com/uploa...1108707573.jpg

I guess these two little items combined were the major contributors to my car's oil consumption, and the reason for the teardown. I'm now into new rings, new valve guides, cyl head rebuilds along with all the other stuff mentioned in my earlier thread.

Any thoughts on why these failed?

LeRoux

i repaired a '87 930 base gasket leak last week, and the gasket looked pretty close to that, without the holes, just pooched out enough to leak a lot. all head nuts tight, no sign of wear of movement anywhere. the engine was apart in the past, so i assume it was damaged on installation. being soft copper, they can get thinned out over the years from thermal expansion, and seep a bit, or in the case of loose heads, the cylinder can shuffle around and cause them to break down. those particular stem seals are difficult to install without killing them, so maybe an installation problem again. the 964 stem seals go on better.

JWW

Thanks for that. The wrench who removed those base gaskets and stem seals supposedly also installed them for the prevous owner, so we'll have some words about that.

I'll go with the 964 stem seals then. I assume they will fit with no other modifications?

LeRoux

they should fit fine. there are two or three different types of machined finishes on the top part of the guide that holds the seal, and some seals fit tighter than others. in the picture, notice the seal, second up from bottom left. the hole is larger than the one below it. the rubber in some seals gets hard and looses it's grip around the valve stem, allowing oil to pass.