Differential Mount Repair

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Courtesy of Peter Florance

Here's what can happen if the problem is not corrected before the frame rail fails completely. If the car feels "springy" when you accelerate, put it on stands and inspect the differential mount carefully.

Here's the basic problem. The rear of the differential is supported by a rubber bushing. The bushing is attached to the sheetmetal unibody box via 4 10mm bolts screwed into nuts welded to inside of the unibody box section. The same torque that causes e12's to 'squat' upon acceleration, applies a pulling force to the back of the differential. Eventually with enough force the sheetmetal fatigues and the nuts start to pull away from the rest of the sheet metal. Mine first failed when the car was about 7 years old. You can see evidence of the mig welding that was done by a local body shop. As the shop did not remove the bushing (I didn't know that at the time) they were unable to weld the cracks completely. There was a repair kit available for this problem but I was unaware of it at the time of the first repair. It is now NLA from BMW but JustBMWS has a few in stock. Karl gave me the idea of supporting the sheetmetal via bolts coming down from the trunk floor and using nuts on the diff mount bushing itself.

Although I had pictures of the bottom part of the repair, I didn't know what Karl's repair looked like on the top. I did know that the rear mounting nuts (the ones that fail) were directly below the rear edge of the sunken in area of the trunk. The edge is at about a 45 degree angle so there's no easy way to just use bolts alone. Karl did use some sort of plate to provide a flat surface.

I designed a plate that would fit in the sunken-in area. It has an 'L' shaped cross section. I'll provide dimensions later (yeah, sure). The plate is shaped and beveled to fit flush in the area.

Here's a closeup of the plate. Note the beveling. It's a good idea to make the plate out of something no thicker than 1/16" if you want to MIG weld it into place as I did. Welding very thick metal to very thin metal is very difficult to do well. I used 1/16". I feel like I should have beveled the top as well to make the welding easier.

Here's the plate welded into place with the bolts (M10-1.5 x 190mm long). Note the brazing on the right side. That's where I melted a hole in the trunk. Doh! What a difference! I'm sure the mount has been hosed for quite a while. The car feels a lot more solid now. The bolts are threaded from the top into the mount holes and then snugged up. Don't tighten too much or you will collapse the box section. Note with the box deformed like mine it was really tough to get the bolts to catch the threads. Since I did this work I found one of the repair kits for the bottom. You could use one of those alone but I'm happier with the idea of both. Once that comes in and is installed I will seal around the bolt holes with seam sealer and rustproof up in the box section.


3/28/2001:  Here are some pictures of Karl Koenig's repair using the factory repair kit and through-bolts (Karl was my source for this method).

06/10/2004:  Here are some before and after pictures of the differential mount in Bob Szlasie's '80 633i. Dinan Engineering did the repair work.


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