Diff bolt upgrade

[m3f_ryivnx_c9ad93070c]

Rebuilding the back end of my beloved M3 and was reading that I should upgrade the diff bolt to a higher grade. This is what people have been referring:



part number 33321090693



(bolt #13 on http://www.realoem.com/bmw/showparts.do?model=BF93&mospid=47486&btnr=33_1004& hg=33&fg=30&hl=32 )



My question is this: Do I need just one of those or two for the diff and is it a direct fit or some modifications are required? Sorry, new to all of this



thanks

[m3f_Roland H_da3d16ded4]

In the diagram you linked:


#13 is the upper control arm bolt.


#14 is the lower control arm bolt.


In this diagram: http://www.realoem.com/bmw/showparts.do?model=BF93&mospid=47486&btnr=33_0371& hg=33&fg=10


#16 is the differential bolt that often breaks.

[m3f_drummer bo_70cb8eddd6]

Pretty sure it's #16 on here:



http://www.realoem.com/bmw/showparts.do?model=BF93&mospid=47486&btnr=33_0371& hg=33&fg=10





And this is the upgraded version:



http://store.bimmerworld.com/mobile/e36-109-grade-front-differential-bolt-upgrade-p1516.aspx

[m3f_fiveightan_7e37700daf]

For further clarification, there are 2 mounting points in the back of the diff, and one in the front. The front diff mount bolt is the one that's prone to breaking.



If the mounts are in good condition and the car make anywhere near stock power, there's no reason why a new stock diff bolt should break any time soon.



Personally, I replace the thing every so often. It takes very little effort and it's under $2 for an OE bolt.

[m3f_badgtho_2cb87af10e]

I bought the upgraded bolt and an OEM one to compare when I swapped in the 3.38. Of

Course I installed the OEM one by accident. Oh well. It hasn't broken yet.

[m3f_ryivnx_c9ad93070c]

Thanks for the replies, guys. So the other 2 (#13 on the diagram Inprobided) not necessary to replace? I'm sure I'm going to have a ton more questions as I amass and proceed with this rebuild. I've read the threads here, hopefully that was a good enough start

[m3f_Roland H_da3d16ded4]

Thanks for the replies, guys. So the other 2 (#13 on the diagram Inprobided) not necessary to replace? I'm sure I'm going to have a ton more questions as I amass and proceed with this rebuild. I've read the threads here, hopefully that was a good enough start



Yeah, those are for the control arms and typically never fail.

[m3f_drummer bo_70cb8eddd6]

For further clarification, there are 2 mounting points in the back of the diff, and one in the front. The front diff mount bolt is the one that's prone to breaking.



If the mounts are in good condition and the car make anywhere near stock power, there's no reason why a new stock diff bolt should break any time soon.



Personally, I replace the thing every so often. It takes very little effort and it's under $2 for an OE bolt.



This is the key factor. In fact in one of the reviews for the upgraded bolt on BW, a guy mentions how he broke a stock bolt and the upgraded one in the same day because his mounts were so shot. The only real need for an upgraded bolt is if you are running solid mounts and/or making a good bit more than stock power.

Thanks for the replies, guys. So the other 2 (#13 on the diagram Inprobided) not necessary to replace? I'm sure I'm going to have a ton more questions as I amass and proceed with this rebuild. I've read the threads here, hopefully that was a good enough start



actually #15. No need to replace them though as they are not prone to breaking like the front bolt.

[m3f_ryivnx_c9ad93070c]

Got it, greatly appreciate the advice:thumbsup2:

[m3f_ckpitt55_5dd6463ed4]

Kind of OT from the op, but kind of not..


Has anyone ever upgraded to a 14mm front bolt? AKG sells a 75D bushing with a 14mm sleeve in it.... PN# #DM36F14. I imagine it wouldn't be all the difficult to drill and retap the case for a 14mm thread and pick up an ARP bolt or something of the right length. I imagine that would be pretty bulletproof, especially if you add something like the Max PSI diff support


That is what I intend on doing.

[m3f_fiveightan_7e37700daf]

Kind of OT from the op, but kind of not..


Has anyone ever upgraded to a 14mm front bolt? AKG sells a 75D bushing with a 14mm sleeve in it.... PN# #DM36F14. I imagine it wouldn't be all the difficult to drill and retap the case for a 14mm thread and pick up an ARP bolt or something of the right length. I imagine that would be pretty bulletproof, especially if you add something like the Max PSI diff support


That is what I intend on doing.

There's simply no need to. You can go through the trouble to do that if you want to mask the consequences of worn mounts. But with properly functioning mounts, you're not going to snap a stock size diff bolt. If you're putting down huge amounts of power, you may want to look into one of the many diff reinforcements out there. But with a NA motor, it's not needed. Drilling and re-tapping the threads will leave arguably less robust threads as well. Then you're adding a stiffer bushing, so you're transmitting more force to the bolt simply by changing to that setup.


I literally beat the living **** out of my diff bolt. I autoX the car on reasonably sticky tires, so it sees tons of very abrupt and harsh on/off throttle stress and constant low speed throttle steering. I drive the car very very aggressively. I'm constantly accelerating from a stop at full throttle, kicking the back end out and sliding it around, banging into another gear. I'm really rough on it. Of the guys that have seen how I drive my car, they'd probably agree that only someone like Bobby is harder on their diff bolt, and he's never snapped one either. We've both had our diffs die...but not the mounting bolt, the bolts were fine. :laughhard:


I've never so much as seen a bend in one of my diff bolts. Granted I do replace them periodically for good measure. But my mounts are in good shape, so I don't expect to snap a bolt. IMO, replace your mounts and use a BW upgraded bolt. At the end of the day, however, do whatever lets you sleep at night. That's just my 2 cents.

[m3f_pbonsalb_ba5494e5cb]

Kind of OT from the op, but kind of not..



Has anyone ever upgraded to a 14mm front bolt? AKG sells a 75D bushing with a 14mm sleeve in it.... PN# #DM36F14. I imagine it wouldn't be all the difficult to drill and retap the case for a 14mm thread and pick up an ARP bolt or something of the right length. I imagine that would be pretty bulletproof, especially if you add something like the Max PSI diff support



That is what I intend on doing.



You can drill and tap for 1/2 inch or for 14 mm if that seems like the easiest solution fir you. The case is strong enough and there are bushings for this. Ideally have the diff out of the car so you can drill straight. Bigger is stronger.



But as others commented, fresh mounts and a stock bolt may be enough. I think an upgraded poly front diff mount with a stock bolt is the way to go, and does not require diff removal (there is room to push the diff back a bit with all mounting bolts removed), for most people.



A diff support brace is another alternative. With one, you will probably be fine with old stock mounts and a stock bolt. Rally Road, AA and Maximum Psi sell braces. If this is easiest for you do this. Max Psi is cheapest but requires welding.



If you have massive power and run R compounds or drag radials and drag race, I recommend doing some or all of the above. I run a Maximum Psi diff support brace, AKG front and rear diff mounts and a longer (due to the brace) stock grade bolt, and have had no issues with 500 lbs rwtq launching on drag radials at the strip -- abuse that has twisted two driveshafts, broken one axle stub, and fused two clutches. Total cost was about the same as just a bolt on brace from AA (welding cost of $30, $30 for the brace and the $100 or whatever AKG charges for the front/rear set). I have run a Max Psi diff support brace on all my diffs -- 3.23, 3.38, 3.64 and 3.15 and never broken a diff bolt despite being supercharged or turbocharged since 2006, and half that time was with old stock mounts.

[m3f_ckpitt55_5dd6463ed4]

everything is coming out for a rear end refresh so I'm doing it all while I'm in there. the car is also getting an ls1, hence my concern for some additional reinforcement. I'll be using either 75D or solid aluminum bushings in the diff to minimize hardware deflection - I've had solid diff bushings in the car for ~5 years with no issues and no noticeable increase in NVH.


Might be overkill but I'm ok with this.

[m3f_pbonsalb_ba5494e5cb]

Should be fun.


Might want to rebuild the diff if it is coming out and you will have loads of torque. The stock diffs are 15-20 years old and there are more and more reports of loose bolts. www.bimmerdiffs.com takes much of the mystery out of upgrading these diffs. I am about to pull mine to replace and loctite the bolts and add a clutch. I'd also do all the reinforcements AKG sells that you do not have or have not already done.

[m3f_ckpitt55_5dd6463ed4]

Should be fun.


Might want to rebuild the diff if it is coming out and you will have loads of torque. The stock diffs are 15-20 years old and there are more and more reports of loose bolts. www.bimmerdiffs.com takes much of the mystery out of upgrading these diffs. I am about to pull mine to replace and loctite the bolts and add a clutch. I'd also do all the reinforcements AKG sells that you do not have or have not already done.


That's the plan - new bearings, 3.91 gears, and a rebuilt LSD. Jury is still out on how I want to configure it. I'm also going to rebuild the axles.


As far as other reinforcement - RTAB pockets and subframe mounts obviously. Will probably end up seam welding critical portions of the chassis as well.