The W211-based E55s have a serious design flaw in the rear subframes.  There have been many reports of the rear subframe bolts snapping in two or ripping their mounting bosses right out of the body.

This typically happens under hard acceleration and can cause a lot of expensive damage to the car.  The rear subframe is held in by four bolts, and it always seems to be the rear two that have the problem, never the front two.  This makes sense, since under acceleration the rear end is trying to squat while the front end rises due to the torque imparted on the chassis by the drive axles.  The front half of the subframe, ahead of the axles, is pushing up into the body, while the rear part, behind the drive axles, is pulling down on the body.  With 516 ft-lb of torque available from even the stock engine, those two rear mounting points tend to fatigue and fail when you least expect it.

Adding the large-diameter washers is the key to this fix.

The fix is pretty straight forward, especially if you do it before it becomes a problem.  It’s a simple matter of replacing the factory rear subframe bolts with a pair of new bolts with nuts and large washers on them, creating a “sandwich” that is much stronger than the original setup.  Adding the large-diameter washers is the key to this fix.  They spread the load out over a much larger surface area than the original mounting bosses, thereby making it nearly impossible to tear them out of the body.  My car has over 180 dragstrip passes on it, all on sticky Hoosier drag radials, and the bolts have held strong.
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