Hey all,
We've broken the front diff bolt a couple of times now ... what's the proper fix for a race car? I guess some sort of double shear bracket. Anyone have pictures of what they did?
Thanks!
Before we start...are you already running solid bushings for your diff and have you already drilled out the diff to allow for running a full length bolt with nut on the end rather than threading the bolt into the diff casing?
Proper fix? Modify the subframe and run an E30 diff case with the E36 diff cover.
Outside of that, there are a couple options you can go with. One is to buy the brace from Active Autowerks. It's heavy, but does the job. Another is to weld on a double sheer reinforcement to the diff case...search for Mike Radowski on the Forced Induction forum.
Last edited by B.Watts; 06-02-2008 at 11:20 AM.
Here are a couple threads with solutions from the FI forum:
http://forums.bimmerforums.com/forum...d.php?t=856579
http://forums.bimmerforums.com/forum...d.php?t=698916
Solid diff bushings, yes. Drilled out bolt hole, no. I guess that's next next stop since I need to pull the diff out anyway to remove the broken piece.
Out of curiosity, what makes the e30 diff case better?
Have they had to re-tap your threads on your diff after the first 2 times it snapped? Or was there no thread damage?
good information, since I've got my diff out now and replacing the bushing. Just ordered a weld on kit from Mike.
Thanks for the info Bryan.
Mines drilled and tapped (which must have been convenient when Watts borrowed it last year) to an american size 1/2", then the casing gets ground down a little bit to accept a nut on the back. Grade 12 hardware. She's been lasting years but Diffsonline WILL NOT want my carrier for a core, kinda ruins it for a normal street car.
Ikes car should be setup with the E30, anyone have pics?
evosport also sells the diff support bracket.. what I have on my car as PM so it never breaks
"Torque is like cowbell... you can never have too much." - Michael Cervi
i broke my diff bolt 3 times before i drilled out my diff and now have a grade 12.9 bolt and nut holding my diff on.
The diff bolt breaks because the original bushing needs to be replaced. The mating surface of the bushing to the diff gets rounded off, and applying throttle causes the diff nose to lift up. The rounded face of the bushing allows the bolt to flex and bend, and eventually it fatigues and breaks.
The solution to 99% of all diff bolts breaking is a new bushing and new bolt torqued to spec. Unfortunately some aftermarket solid diff bushings have an inadequate sleeve inside the plastic, and it crushes under the bolt torque. The bolt loses torque and quickly breaks.
For Watts diff, I drilled and tapped the hole in the nose of the diff to M14x1.5 and used an E46 M3 diff bolt. It's in a solid acetal plastic and stainless steel sleeve bushing. It is torqued 105lb-ft. That sucker is never going to break.
James Muskopf
RRT Racing
DC Metro's premier BMW service and racing facility
like James said, do it right. Replace the bushing and also make sure you are not just retapping it, that is not the correct fix, use a Timesert and do it right. then some blue or red locktite, torque to spec and you will be problem free.
Going to a larger bolt is also a decent idea, just remember the higher the grade the more likely a bolt is to snap vs bend. Stick with 10.9 or 12.9 and you will be fine.
The brackets are nice though if you want to buy one, but we have never used one on any e36 we have raced in house or on any customer's cars.
The brackets do transmit more noise to the subframe when applying throttle. It really caught me by surprise after installing the AA bracket on a customer's turbo M3.
James Muskopf
RRT Racing
DC Metro's premier BMW service and racing facility
Bumping this, what about just reinforcing it with a weld on kit like mentioned? Is that just as good as a support brace? What bushings do you guys recommend for a street/track car. I was thinking the akg diff bushings? Also if I get upgraded bushings is it more of a necessity to have a support brace or is it better to have a support brace with stock bushings? Thanks
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