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.
What you’ll need for this job
Follow these links to purchase the items needed for this project:
Let’s get started!
You’ll want to get the rear of the car up on jackstands. To do this, use a floor jack and a short block of wood (about 8″ of 2×4 works great). The E55 sits pretty low to the ground, so you’ll want to put the airmatic in “raised” mode. Alternatively, if you have STAR you can manually raise the rear by several inches. Or, if you have coilovers, you may need to use a second floor jack at the factory lift points to raise the body high enough to get the big floor jack under the differential. To avoid crushing the aluminum cooling fins, put the wood block on the jack and position it under the iron part of the differential, as shown in the picture. Once you have this in place you can jack the rear up as high as needed to get the jackstands under the factory lift points.
Now remove the two rear tires, and then remove the rear access panels in the fender liners. Each access panel is held on by three 10 mm plastic nuts. Next you need to wrestle a little with the access panel to get it loose from the rest of the fender liner.
At this point you should be looking at sheet metal. You’ll see a plastic access plug right above where the rear subframe bolt goes up into the body. Remove that plug so you don’t cut into it during the next step. Use a hand drill and the 2″ hole saw to cut a circular access port in the body. Place the pilot bit in the little valley in the sheet metal, directly below the small hole with the plug now removed. Begin drilling and keep going until you’re all the way through. I was pleasantly surprised at how easy this was; a simple bi-metal hole saw cut through the body sheet metal pretty easily. Do the same on the other side of the car.
After the access ports are cut, it’s time to replace the rear two subframe bolts, one at a time. Get your E18 external torx socket on a 1/2″ drive ratchet or breaker bar and remove one of the bolts. Keep the other side in-place so the subframe stays tight to the car during this process. Once the bolt is out, you need to get the 13 mm drill bit and enlarge the hole. This is also pretty easy, since you are pretty much just drilling out the threads. Make sure you go all the way through so the bit comes out the top into the cavity you exposed in the previous step.
Make sure you are using class 10 nuts and class 10.9 bolts.
Now get one of your new bolts, a nut, and a washer. Make sure you are using class 10 or stronger nuts and class 10.9 or stronger bolts. (Visit this BoltDepot.com chart for more details) Put the bolt in from the bottom-up, just like the original bolt was installed, only this one is longer so it will extend up inside the cavity. That’s where you’ll put the large 37 mm diameter washer and a nut. Use the box end of an 18 mm wrench to hold the nut while you torque the bolt with a 16 mm socket to 100 ft-lb.
Wrapping It Up
If you live in an area where they salt the roads, you may want to close up the holes you drilled. If you tack-weld the washers and nuts in place you can weld the round slug back into the hole. Then grind it smooth, hit it with a little paint, and replace the rubber plug. This way you’ll be able to remove the bolts in the future without needing a wrench on the nuts. You can also use some RTV to glue a rubber sheet over the holes to keep debris out. Or, you can be like me and leave them open. The fender liner does a pretty good job of keeping muck out, at least on the roads up here in Portlandia.
That’s all there is to it! I did mine over three years ago and have never had to worry about the rear end being torn from the car. I highly recommend this for anyone who wants to put big sticky tires on your E55.