Quote:
Originally Posted by B6T
^ The literature published about the original IMS states that the seals are compromised due to the type of service environment the bearing operates within, the grease washes out, and the lack of sufficient lubrication kills the bearing in a short period of time.
Therefore, the age of the bearing is a factor more than the mileage because as the bearing ages, the seals dry up. The occurrence of failure in low mileage cars supports this theory (more inactivity, less oil circulating around the bearing, etc.). From what I have read, I don't think anyone has seen either a single row or dual row bearing fail based on pure mechanical loading conditions, so it always has to be lubrication related. The fact that Porsche switched from a dual to single row bearing also supports that - and they have the type of design and testing data on the M96 that most engineers would salivate over.
So your statement that the "the moment your car hits the ground after installing it, you have exactly the same probability of failure that you had before you took it apart" is false because you will have refreshed the seals and eliminated any degradation with time that has occurred with the original bearing.
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And I have four customers that had OEM IMS failures in less than 6K miles on new vehicles. While their engines were replaced under warranty, it also says that being brand new does not exempt the OEM style bearing from failure, so it is more than just the seals failing over time. One of the guys that suffered an early fail had the one in the replacement engine go less than a year after it was installed (as soon as he got his second replacement engine, he drove the car to another dealer and traded it in).
As for pure mechanical failures, we (and others) have seen a couple where the center bolt of the IMS was found lying in the bottom of the bell housing. And while the bearing was tore up from the bolt failure, the balls did not show the normal severe galling associated with long term lubrication problems. So there are different ways these things can crap out, and they can a do die young, which I think is one of the factors contributing to the general belief that low mileage cars are more susceptible than those that accumulated a lot of miles quickly.
Porsche's own data in the class action noted that the expected rate of failure for the single row bearing was around eight or so percent, with no caveat for the age of or mileage on the bearings.