Originally Posted by davemoore
Well, Poz, I thought the same way you did. With absolutely nothing wrong with my FD, I swapped out bearings at 53k miles (85.9k km). The second bearing lasted 22k miles (35.6k km) before failing, well under 60-70,000. You never know.
Having rebuilt several drives, I am very curious about these failures of rebuilt drives. You "swapped out" the bearing. Did you check and measure preload using the BMW tool or using Dman's technique with a dial micrometer? Or some other method?
Or did you just disassemble the drive, remove the old bearing and replace with a new one using the original shim(s). Many have done this, and in the early days of FD failures I suspect BMW service departments also did it this way until they got "educated" and started replacing the whole drive. Several BMW rebuilt drives later failed too.
If preload was not calculated when the new bearing was installed, the failure comes as no suprise to me.
If preload was carefully caluclated, and appropriate shims were installed, I wonder what the failure mode was? Did the bearing retainer break as seen in many crownwheel bearing failures?
Also, was the new bearing installed with out excess pressure that might result in spalling of the balls and races?
If a carefully and properly rebuilt drive fails has a crownwheel bearing fail, that points to a cause of failure that is beyond the currently suspected reasons.