E92 M3 broken differential bolt

[kmarei]

Are the bolts in question the 2 that go into the 2 ears coming out of the diff in this picture?

[e92zero]

^elp_jc. I believe zba posted on page 2 that the torque spec is 47 lb/ft. I don't plan on loosening them up first and retightening them. I should be able to just set my torque wrench to 47 lb/ft and see if the nut moves first or my wrench clicks first. If the nut moves first, that should be a good indicator that it's not up to spec. If the wrench clicks without the nut moving, then it's 47 lb/ft or more and I will just leave it as that. I am just concern that the nuts loosen up over time. Anyone want to charm in if the above is not a good idea?

^Kmarei, that is correct.

[elp_jc]

Originally Posted by e92zero

If the wrench clicks without the nut moving, then it's 47 lb/ft or more

You need to review your high-school physics book buddy . Remember it takes a lot more force to start moving something than once it's moving, all depending on the coefficient of friction of both surfaces. If the nut moves, it was WAY undertorqued. If it clicks, it could be undertorqued, but probably not by more than 15% at the most, so not a huge deal. But if you want it torqued correctly, only way to do that is to loosen it first then retighten it .

[e92zero]

^ haha. got it. I guess I wasn't clear but thanks for pointing out the detail. But bascially if my wrench clicks before the nuts move, I understand it can still be off spec either side but I am not going to loosen it and retighen it as it's "factory" spec so if it still fails that way down the line, it's bmw waranty. Don't want to run the risk of them claiming that I loosen up the bolts to cause the failure.

On another side note, the last time I checked under my car, the 4 subframe bolts started moving when my torque wrench was at 59 lbs. I just found out that the torque spec on these 4 nuts are suppose to be 73.7 lbs (100NM) on the regular 3 series. I am assuming the M3 should have the same torque spec at 73 for these 4 bolts. Anyone got info on these? I am going to check these also this weekend and report back. but off by almost 20%?!? Not encouraging

[Zba]

Originally Posted by e92zero

I just found out that the torque spec on these 4 nuts are suppose to be 73.7 lbs (100NM) on the regular 3 series. I am assuming the M3 should have the same torque spec at 73 for these 4 bolts. Anyone got info on these?

Yes, 100NM on all bolts for the rear sub-frame to the body

[e92zero]

^thanks Zba. Then i guess mine are all under torqued at 59 lbs.

[kiln]

I have a 2011.75 6mt sedan. After reading this thread i decided to go check the differential and all 3 bolts fine. I also have an aftermarket exhaust installed. I have never had any excessive wheel hop abuse either.

[e92zero]

Reporting back. The diff nut is under torqued on mine on the driver side. The nuts start moving at around 35 lb/ft. However, I can't put my torque wrench at the passenger side nut though so will get a crowfoot before I tighten up both side.

The subframe bolts are also under torqued at around 59 lbs.

Car is a 2011.00 E92, 6mt. 22560 miles on the odometer.

Never launched my car, actually I don't remember ever taking off really fast off the line. 95% freeway driving. I would really like to know if these bolts were not tightened up to spec at the factory or they came loose over time, or the washer compresses over time. A bit discouraging none the less. I guess I will check them again in another 10k miles or so.

[bigjae1976]

In the cases of broken bolts...is the diff bushing still good? Are the dealers replacing them?

The subframe issue in the E46 was caused by worn subframe bushings. The bushing would excessively twist and load up the mounting point which would eventually fatigue the metal.

Maybe BMW improved the subframes and bushing design in the E9X but now created the problem with the diff bushings?

Maybe the heat from the exhaust (sans the heat shield) cause the diff bushings to fail from heat?

[Singletrack]

Originally Posted by Zba

47 ft/lbs.

Does anyone know if these bolts are aluminum?

I also had a clunking from the rear end and found my two smaller diff bolts loose, not enough to move them by hand, but not torqued to spec. If the bolts are aluminum, that could certainly be an issue (as far as breakage) if the proper torque is not used.

Originally Posted by Zba

Yes, 100NM on all M12 bolts for the rear sub-frame to the body

I'm confused as to which bolts get which torque. I might just sign up for the online bmw resource to figure this out.

[Zba]

Ok, I re-read my post. Without having my car on a lift, I am not certain as to what size the bolts securing the sub-frame to the body are, but those bolts are torqued to 100 nm or 74 ft/lbs

The smaller of the bolts which break on the rear differential are torqued to 100 nm or 74 ft/lbs as well.

The large diff bolt is torqued to 165 nm or 122 ft/lbs.

The torque I had previously posted was for non M cars. My bad and sorry for any confusion.

[e92zero]

^Zba. Thanks again for the torque spec.

I just read the article in the Motortrend magazine that talked about the new M5 rear diff. Here is the part that caught my attention:

"In the case of the differential, it's mounted solidly-aluminum to steel, with shear panels to further stiffen it. This is similar to what is done in the M3 GTS."

I couldn't find pictures or information on the diff of M3 GTS. Anyone know if the M3 GTS diff has re-enforcement or it's exactly the same as the regular M3?

[Malek]

Originally Posted by e92zero

^Zba. Thanks again for the torque spec.

I just read the article in the Motortrend magazine that talked about the new M5 rear diff. Here is the part that caught my attention:

"In the case of the differential, it's mounted solidly-aluminum to steel, with shear panels to further stiffen it. This is similar to what is done in the M3 GTS."

I couldn't find pictures or information on the diff of M3 GTS. Anyone know if the M3 GTS diff has re-enforcement or it's exactly the same as the regular M3?

I'm glad you posted this.

Putting the solid rear bushings will prevent damaging of the subframe and reduce if not eliminate all system compliance.

It is a counter intuitive change as many always believe that a solid mounted rear end will destroy the subframe and unibody mounts. But if done correctly, it will further improve it hence the reason the GTS and new M5 have it.

-Malek

[e92zero]

^Malek,

I might have been confused, but are we talking about solid bushings between the diff and the subframe, or solid bushings between the subframe and the chassis/unibody?

I would imagine solid bushings between subframe and chassis would transfer a lot of shock and vibration to the cabinet which might not be desirable for a street car.

We are all speculating here and the culprit might just be that these nuts are not torqued to spec on a Friday afternoon in the factory in Germany. :P However if these bolts are indeed giving out because of movements of the diff, wouldn't these movements/stress be absorbed by the ear/arm (the mounting tabs) of the diff and the mounting point of the subframe if we were to use solid bushings between the diff and the subframe? I am no expert on this subject though so just guessing.

[Malek]

Originally Posted by e92zero

^Malek,

I might have been confused, but are we talking about solid bushings between the diff and the subframe, or solid bushings between the subframe and the chassis/unibody?

I would imagine solid bushings between subframe and chassis would transfer a lot of shock and vibration to the cabinet which might not be desirable for a street car.

We are all speculating here and the culprit might just be that these nuts are not torqued to spec on a Friday afternoon in the factory in Germany. :P However if these bolts are indeed giving out because of movements of the diff, wouldn't these movements/stress be absorbed by the ear/arm (the mounting tabs) of the diff and the mounting point of the subframe if we were to use solid bushings between the diff and the subframe? I am no expert on this subject though so just guessing.

I will be back in a couple hours with a more detailed reply. I have some Solidworks stuff I need to finish up right now, but I will be back. This is an important topic on this forum.

[bigjae1976]

I installed poly subframe bushings in my E46 and there is no discernible increase in NVH. I think the biggest NVH adders will be any poly or solid driveline mount.

[Ateam]

I read the whole thread and perhaps I am dense but could some tech whiz be kind enough to post some pics with labels of which are bolts needin 47 ft/p v. the ones needing 79 foot pounds?

[Kyoshi71]

When installing the exhaust hangers, it would be important to know if a load on the bolt at the time of people re-tightening the nut (or the bolt for that matter) would have any impact.
If your car is jacked up from the body and the rear is hanging, torque fators on the bolts may be compromised due to the weight.

[Zba]

Originally Posted by bigjae1976

I installed poly subframe bushings in my E46 and there is no discernible increase in NVH. I think the biggest NVH adders will be any poly or solid driveline mount.

Agreed. Solid subframe bushings where there is little load transfers will have a minimal effect on NVH

[kmarei]

checked mine today
all tight and snug

[Zba]

I updated my earlier post regarding torque specs. I had posted the specs for a non M car. All is the same except the torque on the smaller diff bolts which is more on the M's compared to what I had posted.

The smaller of the bolts which break on the rear differential are torqued to 100 nm or 74 ft/lbs as opposed to the 47 ft/lbs I posted earlier.

The large diff bolt is torqued to 165 nm or 122 ft/lbs.

[elp_jc]

Originally Posted by Zba

The smaller of the bolts which break on the rear differential are torqued to 100 nm or 74 ft/lbs as opposed to the 47 ft/lbs I posted earlier.

You should post them with pics, for those who haven't been under the car. And yes, the differential is only held to the subframe by 3 bolts.

Hey, do you have the torque specs for the NUTS that hold the exhaust brackets in place? The differential 'ears' are reportedly threaded (Malekreza11, I believe), but I haven't confirmed this myself. If they are, there should be a (lot lower) torque value for the nuts mentioned in the service manual. Thx.