I was going to do a little write up on this, but I will post a little bit of info in here.
This happened to me on my 2011 E92 M3.
Differential bolt snapped and sheared off, the differential fell down to one side and bent the subframe differential mounts.
Took car into the dealership, they told me to take a hike. I called BMW NA, they gave me some trouble too. I wrote a technical letter to them and one of their engineers explaining that this is not normal under any circumstance, especially since they were blaming the problem on a non OEM exhaust system (doesn't make any sense).
After 2 weeks of hassle and going back and forth with my car sitting there doing nothing, they finally decided to do the repairs on their own bill. The replaced the subframe, the differential and CSB. (All brand new)
They didn't do a proper job of aligning the subframe (rear axle carrier to be proper) into the unibody. I had to create a tool which centered the RAC to the unibody and center it. They also installed one rear spring upside down in which I corrected myself quickly.
I urged them to send the failed parts back to BMW in Germany for analysis, and they did. I also wrote another technical letter to BMW AG explaining the situation and giving possible improvement for the RAC. So far I have been told that the bolt was possibly defective but I felt that this was highly unlikely and it was do to compliance in the subframe differential tabs.
During this situation, I have analyzed the rear subframe myself, and have decided to design a rear subframe reinforcement kit and offer it to the public in due time.
BMW PERFORMANCE SPECIALISTS. Race Engines. Suspension. F/I. Brakes. Race Preparation. Factory Service. Alignments.
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