924Board.org

[rOOkie]

During my 81 931 exhaust system rebuild I took off the oil manifold and found that the sealing surface on the block is very rough. It appears to be factory machining marks. Has anyone else seen this?

My opinion is that a oring will have a more difficult time sealing small bumps than a surface that is not perfectly flat (planar).

Thus, I was planning on plugging the holes and taking a sandpaper block to the surface and smoothing it out. Good idea/bad idea?

I have a lot of oil on this side of the pan/block/head and suspect that this may be the culprit.
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81 931 - A gift that keeps giving....

[gohim]

Do not smooth out the sealing surface if it was originally left with a rough surface during machining/manufacturing. The sealing surface was made this way on purpose by the engineers to help seal.

[CMXXXI]

"Oil manifold"? Are you speaking of the adapter that holds the oil filter and lines to the oil cooler?
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'79 Eurospec 931

[rOOkie]

Oil manifold is referring to the adapter for the filter and cooler lines.

Normally I would agree that a "roughened" sealing surface might be intentional...but I have a hard time believing that Porsche engineers would design a seal surface with scratches (grooves). The grooves run across the o-ring landing area and are significantly deep. I also see similar grooves on the engine support bosses on the block which make me believe they are not sealing specific. Photo to be attached shortly...

My main question is: Does anyone else have this on their block? Or am I just lucky?
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81 931 - A gift that keeps giving....

[rOOkie]

a not so good picture...

http://members.shaw.ca/geoffcrocker/pictures.htm
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81 931 - A gift that keeps giving....

[CMXXXI]

Those are without question just tool marks from the machining process. If they feel slightly rough/raised I wouldn't worry about it. You'd probably do more harm than good if you tried to manually smooth the surface out. Short of taking the block to a machine shop I'm not sure really how much you could do anyway...
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'79 Eurospec 931

[rOOkie]

The grooves are probably about 0.010" deep. I would consider that to be significant...I know from o-ring catalogs that the mating surface of a face seal should have a peak to peak roughness of 0.00064".

Maybe a better question to ask is: Has anyone taken their oil manifold off and is the surface on the block smooth like a babies bum or rough like an old man's hands?
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81 931 - A gift that keeps giving....

[CMXXXI]

If it weren't getting an adequate seal, the issue would have manifested itself a long, long, long time ago. I find it difficult to accept that a block would have left the factory with that entire face machined so roughly that it would't seal as engineered. When I rebuilt my engine I didn't remove that manifold because it 1.) wasn't leaking and 2.) I couldn't break the bolt loose with "ease" using a large 3/8" drive ratchet. Sorry I don't have photos or a recollection of the smoothness of that machined surface.

FWIW, the smoothness requirement of the sealed surface will vary depending on (among other things) the O-ring material used, how the ring is confined & allowed to compress/expand into/against the sealed surface, and force applied.
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'79 Eurospec 931

[Jazzerpet]

To answer you question yes I have had a few of these off. The sealing surface is just a bit rough. I will say you should replace that o-ring with a new one no matter how good the old one looks. The old one looks to be a flat oblong hard rubber ring but you will find it started out as a normal round o-ring. Pay close attention to the torque specifications for all the parts of that assembly. Every time I have had a leak after a rebuild it was there. Jim
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1980 931
1985 ½ 944
Member of the “Omaha Crew”