Just found the source of the rear end clunk in my MS6. Pulled the e-brake up one click and the clunk went away when the drivetrain was only turning the front wheels. Got under the car and found the front diff mount sheared off the bolts. One bolt was missing, the other had about 2mm sticking out of the diff housing so I could back it out. My car is completely stock (no power adding mods), 37k miles, and above 6k' elevation (reduced power).
Aluminum has a higher coefficient of thermal expansion than steel, this combined with vibration, shock, and driveline lash leads to the bolts losing torque and backing out. The bolts will not shear if they are tight since the load generated by the bolt torque is transferred into the flat interface between the diff housing and mount. Bolts are not intended to be put into shear. Once the bolts are loose, the load puts the bolts into shear, breaking them. In my case it looks like once one of the bolts fell out, the second bolt became loose and had to take all the shear load = failure. The engineer responsible for this part on the car should have speced thread locking compound for this joint. After searching the forum, Im not surprised how many people have had this problem. Some users say it will only happen if the car is driven hard, launched hard, has more power, etc. I dont think those statements hold true.
The bolts are a M12x1.25. Metric bolts dont come in grade 5 or 8, but follow ISO 898 standard. If you arent buying the bolts from the dealership, try to find them in grade 10.9 or 12.9. When re-installing the bolts, use Loctite 242 (medium strength blue).
Called the dealer yesterday and was told my car is still under warranty. Have an appointment on Tuesday to have them replace the bolts and check for other damage. I found some temporary bolts to keep the car drivable for the next few days. Once the factory bolts are put back in Ill later add Loctite to make sure the bolts dont back out again.
