[JFP in PA]

Originally Posted by piper6909

I read somewhere there's a school of thought that the way some halves were machined can cause IMSB failures and that if it hasn't failed after so many miles, then the probability of failure is low. I don't subscribe to it, I only mention it as the only possible justification for your what mechanic said.

I think you are referring to the RMS seal failure issue, which was the result of two failure types:

1. The original seal was a Viton based unit that did not have the life expectancy and was replaced by a PTFE based seal, which worked.
2. Out of round case openings which would leak regardless of the seal material used. These were the result of machining issues and Porsche released a "concentricity" test tool that was a go/no go test that was used if the engine was still under warranty; if it failed, the engine was replaced as it could not be corrected in the car.

There was also a non concentric IMS shaft problem, where the shaft would wobble because the IMS bearing center line the the shaft center line were not the same. This was mostly in the early 986 engines and was enoght of a problem that engine rebuilders had problems finding enough usable shafts to reassemble the engines, but that had nothing to do with the engine cases, it was a shaft assembly problem.