E92 M3 Diff Bolt Failure -> Subframe Replacement

[Scandinavian_Flick]

So, I thought I did pretty well by finding a 30k mile M3 back in December, but it looks like the car gods still dislike me for some reason.

While driving home from a friends house I got on the throttle and heard a *Tink* noise from the rear end of the car, but didn’t notice anything drastically wrong with the car at the time, and so I nursed it along the ~3 miles back to my house. When I jacked the car up the following day I poked around the rear end and found one of the front diff mounting bolts to be broken clean off…..



From what I’ve seen this isn’t a common issue by any means, but there’s definitely a good handful of people that have had it happen. Unfortunately, in my case the movement of the diff allowed for the remaining piece of the bolt to smash around in the subframe bushing and ended up causing some pretty severe damage to my subframe. I confirmed with a local performance shop what I thought to be the case; replacement subframe needed. Its feasible that the original subframe could be fixed, but better to get a replacement and know 100% that it’s solid.



With that, I knew that the subframe from a donor vehicle wouldn’t be as sharp as the one coming out of my clean (although broken) 30k mile car, so I knew that I would want to replace the bushings with new. It took some serious convincing from a friend of mine to sway me, but eventually I decided to go with solid subframe mounts instead of replacing with new stock mounts. Eventually I figured that if the subframes in the M3 GTS, F10 M5, and F80 M3 were solid mounted, the increased NVH couldn’t be that bad. I also decided to upgrade to moonball bushings for the trailing arm and will upgrade that arm in the future.

Here’s a list of everything that went into the car:
- Bimmerworld solid subframe mounts
- Bimmerworld rear trailing arm bushing monoballs
- New stock camber arm bushings
- New stock differential bushings
- New stock subframe bushings and brackets
- New mounting hardware and clips
- Stainless steel brake lines (front as well to match)




First I went about burning out the bushings from the donor frame before stripping/scuffing the paint and adding a few new coats of black paint and clear coat before pressing in the remainder of the bushings. Heres some shots of everything installed on the subframe, ready to go in the car:

















Here’s the tricky bit; I wasn’t able to find anyone who really addressed the issue of ‘aligning’ the subframe in the car. My local performance shop (who runs a highly accredited race team mind you) told me that they don’t bother aligning the subframe in the car, but rather just bolt it up and fix any inconsistencies in the wheel alignment process. What!?!?! – Ok ok, I’m sure that’s fine, but I figure since I have the time to make it perfect, let’s spend the time and get it as close to perfect as possible.




I was able to get the subframe in the center of the car I believe pretty accurately by getting out a ruler and measuring the gap on each side from the mounts. I got the rear two mounting points centered within about a millimeter or so, and the front centered within a half millimeter – Perfect. Here's photos of me measuring the front side below.




Re-assembled the rest of the car and took it for a test around the block, and it feels good, went back and checked the torque on all the bolts and all was good. The car needs a good alignment; as of right now, it’s riding on my eyeball spec alignment that I didn’t even do at ride height, although in an oddly worrying way, it drove pretty straight lol.

I’m going to make another post reviewing the solid subframe bushings and the monoball RTABs soon here, and I'll link it here in an edit once I post it, so if you don't see it here yet, just hang tight.

Thanks for reading - feel free to ask me any questions if you have them!

[Ajolives]

Originally Posted by Scandinavian_Flick

So, I thought I did pretty well by finding a 30k mile M3 back in December, but it looks like the car gods still dislike me for some reason.

While driving home from a friends house I got on the throttle and heard a *Tink* noise from the rear end of the car, but didn't notice anything drastically wrong with the car at the time, and so I nursed it along the ~3 miles back to my house. When I jacked the car up the following day I poked around the rear end and found one of the front diff mounting bolts to be broken clean off…..

Attachment 2825946

From what I've seen this isn't a common issue by any means, but there's definitely a good handful of people that have had it happen. Unfortunately, in my case the movement of the diff allowed for the remaining piece of the bolt to smash around in the subframe bushing and ended up causing some pretty severe damage to my subframe. I confirmed with a local performance shop what I thought to be the case; replacement subframe needed. Its feasible that the original subframe could be fixed, but better to get a replacement and know 100% that it's solid.

Attachment 2825951

With that, I knew that the subframe from a donor vehicle wouldn't be as sharp as the one coming out of my clean (although broken) 30k mile car, so I knew that I would want to replace the bushings with new. It took some serious convincing from a friend of mine to sway me, but eventually I decided to go with solid subframe mounts instead of replacing with new stock mounts. Eventually I figured that if the subframes in the M3 GTS, F10 M5, and F80 M3 were solid mounted, the increased NVH couldn't be that bad. I also decided to upgrade to moonball bushings for the trailing arm and will upgrade that arm in the future.

Here's a list of everything that went into the car:
- Bimmerworld solid subframe mounts
- Bimmerworld rear trailing arm bushing monoballs
- New stock camber arm bushings
- New stock differential bushings
- New stock subframe bushings and brackets
- New mounting hardware and clips
- Stainless steel brake lines (front as well to match)

Attachment 2825956
Attachment 2825954

First I went about burning out the bushings from the donor frame before stripping/scuffing the paint and adding a few new coats of black paint and clear coat before pressing in the remainder of the bushings. Heres some shots of everything installed on the subframe, ready to go in the car:

Attachment 2825952

Attachment 2825966

Attachment 2825953

Attachment 2825957

Attachment 2825969

Attachment 2825958

Attachment 2825960

Attachment 2825961

Here's the tricky bit; I wasn't able to find anyone who really addressed the issue of 'aligning' the subframe in the car. My local performance shop (who runs a highly accredited race team mind you) told me that they don't bother aligning the subframe in the car, but rather just bolt it up and fix any inconsistencies in the wheel alignment process. What!?!?! – Ok ok, I'm sure that's fine, but I figure since I have the time to make it perfect, let's spend the time and get it as close to perfect as possible.

Attachment 2825970
Attachment 2825962

I was able to get the subframe in the center of the car I believe pretty accurately by getting out a ruler and measuring the gap on each side from the mounts. I got the rear two mounting points centered within about a millimeter or so, and the front centered within a half millimeter – Perfect. Here's photos of me measuring the front side below.

Attachment 2825963
Attachment 2825964

Re-assembled the rest of the car and took it for a test around the block, and it feels good, went back and checked the torque on all the bolts and all was good. The car needs a good alignment; as of right now, it's riding on my eyeball spec alignment that I didn't even do at ride height, although in an oddly worrying way, it drove pretty straight lol.

I'm going to make another post reviewing the solid subframe bushings and the monoball RTABs soon here, and I'll link it here in an edit once I post it, so if you don't see it here yet, just hang tight.

Thanks for reading - feel free to ask me any questions if you have them!

Damn good work - love the attention to detail. For the record, I don't think you're too far off base with centering the subframe, I'd probably do the same - if I'd get off my lazy ass and install mine.

[Redd]

Nice work! Curious why the diff bolt would snap at such a low mileage. Do you drag/drift the car often?

[Scandinavian_Flick]

Originally Posted by Ajolives

Damn good work - love the attention to detail. For the record, I don't think you're too far off base with centering the subframe, I'd probably do the same - if I'd get off my lazy ass and install mine.

Yeah it really didn't take me any time to get it "perfect" maybe only 5 min or so to measure and adjust it.

[Scandinavian_Flick]

Originally Posted by Redd

Nice work! Curious why the diff bolt would snap at such a low mileage. Do you drag/drift the car often?

Originally Posted by Redd

Nice work! Curious why the diff bolt would snap at such a low mileage. Do you drag/drift the car often?

I only picked the car up in December and have only put like 1500 miles on it. I'm probably more aggressive than most, but not in the form of drifting it or dragging the car. Plus these cars should realistically be able to handle some abuse like that.

I'm not really sure what the crux of the issue was, but the bushings that came out of the subframe were really bad, so perhaps they were just allowing way too much driveline slop which allowed the diff to move enough to stress the bolt. The combo of solid subframe mounts and new diff bushings should really help to lock down the driveline in the car and prevent this from potentially happening again.

[amrazM]

This is more of a prevalent issue on the vert, likely due to the lack of rigidity in the body itself.

None the less, good call on replacing the subframe I would’ve never attempted to repair a damaged diff mounting point cause one you put heat to it it’s never at strong as it was designed to be.

[k_mann]

Nice work! I like the bigger lip the bw bushings use.

In regards to subframe alignment, I personally didn't do it. The front two go into index posts and there really wasn't any play. Plus who's not to say no matter how "even" it is by your measurements it didn't offset it more? I think there's such minimal play it really doesn't matter. That's what adjustable suspension components for alignments are for

[Scandinavian_Flick]

Originally Posted by k_mann

Nice work! I like the bigger lip the bw bushings use.

In regards to subframe alignment, I personally didn't do it. The front two go into index posts and there really wasn't any play. Plus who's not to say no matter how "even" it is by your measurements it didn't offset it more? I think there's such minimal play it really doesn't matter. That's what adjustable suspension components for alignments are for

I understand where you're coming from, but at least in my case I was still able to move the subframe side to side by about a centimeter even though it's indexed on the front. While the alignment can most definitely dial out any discrepancies, I'd rather avoid having to adjust the front end drastically to accommodate for a bad thrust angle. Either way, the car drives great right now, and all elements of the alignment were perfect (well the front needs way more camber but its maxed out currently).

[Drifter2090]

Originally Posted by Scandinavian_Flick

I understand where you're coming from, but at least in my case I was still able to move the subframe side to side by about a centimeter even though it's indexed on the front. While the alignment can most definitely dial out any discrepancies, I'd rather avoid having to adjust the front end drastically to accommodate for a bad thrust angle. Either way, the car drives great right now, and all elements of the alignment were perfect (well the front needs way more camber but its maxed out currently).

Glad to hear you got it all resolved. What're tour thoughts on the solid subframe bushings compared to oem?

[Scandinavian_Flick]

Originally Posted by Drifter2090

Glad to hear you got it all resolved. What're tour thoughts on the solid subframe bushings compared to oem?

I'll be making another post in the future that I'll link here (probably this weekend, this week is insane for me). In summary, they're not too harsh, and definitely make the rear more planted. If you're considering between OEM, then I would definitely consider your use case first, if its purely a daily driver that doesn't see any track time, then the tradeoff probably isn't worth it. I'm young and willing to put up with some harshness in favor of higher performance since my car sees hard driving pretty regularly, and in the future I plan on doing anywhere up to 10 trackdays a year.

[Mog-10]

I had used an exhaust expander to round out a crushed front diff mount it was easy and did it with it still installed took less then a few minutes.

[Scandinavian_Flick]

Originally Posted by Mog-10

I had used an exhaust expander to round out a crushed front diff mount it was easy and did it with it still installed took less then a few minutes.

Originally Posted by Mog-10

I had used an exhaust expander to round out a crushed front diff mount it was easy and did it with it still installed took less then a few minutes.

Glad that was able to work for you, however that would not have been viable in this case.