Differential mount and trunk floor cracking
The rear differential hangs from a mount on the trunk floor, and the torque of the M engines can crack the mount's ears and pull apart the spot welds above it; left alone, the floor can tear. It is the most common complaint about these cars at NHTSA: more than 60 of the 81 filed for M Roadsters and M Coupes describe it. The fix is a welded reinforcement kit, a job specialists quote at 25 to 30 hours of labor.
How to check
Have the car inspected on a lift for cracks at the corners of the differential mount and in the floor around it, lift the trunk carpet and check that the spot welds along the rear floor are flat and intact, listen for clunks from the rear when shifting in first and second, and ask whether a reinforcement kit has been fitted.
