LaJolla rear differential reinforcement

JFENG

JFENG

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Guys, I just noticed that Bimmerdoc has a rear body diff reinforcement that bolts in (e.g. no welding).
Anyone install one of these yet? Seems like the easiest upgrade when I change my rear diff.

John
 
Seems like multiple folks on this forum have reported problems with the body tearing at the diff mount. That's why multiple E9 specialists sell a reinforcement kit. My car, if I ever get crackin on it, will hopefully have +20% torque than original and I plan to drive it pretty aggressively (e.g. rallys and trackdays, no relaxed highway cruising for me). So, if there is a bolt-on reinforcement for just $65, why not?

John
 
Having reviewed several threads about diff mount reinforcement, it's on my list for when I have the car up on the lift for rust repair. I have a nice TIG setup.

I'm not upgrading my 2.8L engine, nor is my mount failed, but years ago, I found a number of fatigue cracks in the frame head of the '54 beetle I was restoring. It had many miles on it, but had only been run on the road with stock engines, yet the strongest part of the pan, where the front end attaches, had cracks several inches long running down the center tunnel. They were easily welded since I had the car apart.

Our cars have a lot more power and twice as many years on them as my long departed VW and there have been failures of the E9 diff mount. I'm going to add some welds to a number of panels since it's all gotta be painted anyway. There's a good series of photos on body and diff mount reinforcement on the coupeking.com site.

As to the efficacy of a bolt-on reinforcement vs. welding, I don't have any clear data. More steel there will help spread the load and keep the bottom of the mount from flexing as much and the small 'ears' on the side will spread the load to the body a bit, but as a pull-it-out-of-thin-air guess, you wouldn't get much more than 20% improvement in stiffness with a bolt-on plate. You still have the issue of the actual attachment to the body, which is via spot welds.

I'd say it's more effective than increasing your horsepower by putting on racing stripes, but if you have a larger than stock engine or have a heavy foot, I'd have it welded.

Ian
 
Mine cracked, made up a frame using 2mm plate and welded it in using a combination of plug and stitch welding,it won't crack again.

John
 
My diff mount was also cracked, and with long cracks going into the transmission tunnel under the back-seat aswell. We removed almost all of the diff mount-box and put in (overkill yes) 4mm plate instead. There are some pictures in my project thread.
My fear is that the only thing this achieves is to put all the stress somewhere else, causing new cracks somewhere else. But I don't have any intention of tracking or racing the car so I'll just keep my fingers crossed.

Cheers,
 
I did the weld in reinforcement. I'm not familiar with the bolt in solution. I hope no one is

Bmw recognized that they had a problem with the diff mounts on the 6 cylinder cars e3, e9, e12. They came up with a solution which I believe Carl Nelson and Paul Cain improved.

I recommend this on all coupes since even stock cars can experience diff mount failures due to fatigue.

Alan, I strongly recommend this for your coupe since you drive spiritedly by your own admittance. That and the s38 power plant.

As for the bolt in remedy- the holes in the reinforcement are for welding, not bolts. Drilling holes would just weaken the area.
 
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Ok guys,
Thanks for all the feedback.
I thought this new part from LaJolla was bolt-in but I see I misread the description.
Well, as long as the rear end is coming out anyway, I should MIG this $65 plate into the car (I hate welding upside down).
John
 
Ah, I looked more closely at the photo of the simmerdoc reinforcement. In addition to the ears and 90 degree flanges for welding attachment, it has holes in the side for welding with four larger holes around the big one for the diff mounting bolts to pass though.
It sounds like the other version has nuts welded behind the holes to replace the damaged ones missing from your car.
They both weld in, one has nuts, one doesn't.

One of the other things on my list to weld up are the tops of the rear shock towers. There have been some failures, some after the change to stiffer shocks.
When the tops fatigue fail suddenly, they blow out the elephant skin covering them.
Welding a large washer to the tops is the answer.

Ian

diff box reinforcement e9 coupe.com.jpg
 
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wow
thats amazing , i had no idea that was a weak point .
my car is really strong there
i think it was babied for all its live
as there was zero rust when i got it
thanks mike ill do it for sure.
so this sits on top of the existing?
 
I have welded in a number of these when I worked at the dealership over 20 years ago, most of them were on E24's and E9's. There was also a second plate that was welded on the back side of the support going from L to R as an extra support. On a few of the cars I had to straighten and weld up the cracks first. These came from BMW themselves so I am not so sure that anyone other than BMW had input into these. It really did help

Thanks, Rick
 
oh!!!
this this goes to the rear mounting?
i thought it went to the front area, where all the
eight mounting bolts exist.
 
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Alan I can't find any pictures. Make sure the 4 bolts for the differential mount line up so you don't have any alignment issues when you go to install the mount.
 
I wouldn't think so. I think your best bet would be to remove the diff and get it out of your way so you can align all the holes and weld up the reinforcement.
 
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