Rear end issues

[169mph]

deleted

[169mph]

I think I found the issue. Most everyone told me to check the the front differential bushing bolt. I checked to see if the bolt was there snd if it was make sure it was tight. The bolt was still there and didn’t seem loose. I tried to torque to spec but it would not get tight enough. I took the weight off the differential with a breaker bar and the bolt basically fell out. When I removed the pry bar the differential dropped back to its normal position and the output flanges are no longer rubbing the subframe. What should I do next? Install a new bolt? Replace the bushing and bolt. The bolt doesn’t look like it’s been sheared off to me. I really appreciate everyone’s help to this point. Any advice moving forward would greatly appreciated. Thanks

[spdbrake]

Yes it is sheared. The clue is the hang-nail thread where it snapped. Machine bolts and screws area chamfered 360 on the end to prevent cross threading. You also need to add the missing fender washer (large area washer under the head)
Your earlier pic:



which is also allowing the diff to shift aft due the two aft mount bolts being loose. You will have to drop the diff out of the car to extract the broken bolt.

While its out you might as well go in with Poly bushings
https://www.ecstuning.com/b-ecs-part...02447ecs04kt2/
https://www.ecstuning.com/b-ecs-part...02447ecs01kt1/
You don't have to go with ECS parts there are plenty of alternative brands for these items.

If your going to James Yale's shop let him advise you and procure the parts for you.
Nothing worse than bringing your box of parts to a shop and expecting them to assume all the risk. "Bringing your own eggs to Denny's" is the common term.

Another thing I'd ask James Yale while its in his shop is upgrading your front diff bolt to 14mm VS the stock 12mm
https://www.ecstuning.com/b-condor-s...me36diffbu~cd/

[169mph]

The crazy thing is when I was turning the bolt I thought I saw the opposite end of the bolt turning to. What I saw turning was actually the sheared bolt if that makes sense. The differential had shifted up about an inch with the broken end of the bolt. Because bolts were so out of alignment I thought there were two separate bolts. Now that the differential dropped back down it’s obvious. My mistake. I don’t like having to drop the differential extract the bolt. Has anyone done this job without dropping the differential? I need to replace the differential cover gasket too so I probably should do what spdbrake is recommending. Thanks again

[spdbrake]

If you're sticking with the stock forward bolt I'd get the improved BMW OEM bolt. https://www.ecstuning.com/b-genuine-...t/33176760337/

Also if you are considering poly bushings and adjustable lower control arms now would be the time to do it. The Diff has to be slid back 2-3 inches and the axles disconnected to remove the upper and lower inboard control arm bolts.

I just reread the last post. "The crazy thing is when I was turning the bolt I thought I saw the opposite end of the bolt turning too". --- Yes broken bolt can be extracted from the rear of the bolt hole without removing the diff.
If you have a Dremel and a cut-off wheel, cut a slot in the protruding tip of the bolt. Then use a screwdriver to walk it out the back of the bolt bore.

[169mph]

I decided to drop the different myself and replace the broken front differential bolt. I’m to pull the differential. The only thing left are two rear differential bolts. Took some time and large cheater to break them loose. I know Im only half done but so far it’s been a fairly easy job. The only issue I had breaking the bolts loose on input drive shaft (4 bolts). I had use an open end wrench to remove them. I couldn’t fit a socket or use a cheater pipe. I have lift I know dropping the differential without one would be way more difficult. I’m plan to upgrade the front differential bolt and replace the three bushings. Is there anything else I should do while have differential our? I really appreciate the help. Thanks Mike

[SBSerpent]

Originally Posted by spdbrake

Yes it is sheared. The clue is the hang-nail thread where it snapped. Machine bolts and screws area chamfered 360 on the end to prevent cross threading. You also need to add the missing fender washer (large area washer under the head)
Your earlier pic:



which is also allowing the diff to shift aft due the two aft mount bolts being loose. You will have to drop the diff out of the car to extract the broken bolt.

While its out you might as well go in with Poly bushings
https://www.ecstuning.com/b-ecs-part...02447ecs04kt2/
https://www.ecstuning.com/b-ecs-part...02447ecs01kt1/
You don't have to go with ECS parts there are plenty of alternative brands for these items.

If your going to James Yale's shop let him advise you and procure the parts for you.
Nothing worse than bringing your box of parts to a shop and expecting them to assume all the risk. "Bringing your own eggs to Denny's" is the common term.

Another thing I'd ask James Yale while its in his shop is upgrading your front diff bolt to 14mm VS the stock 12mm
https://www.ecstuning.com/b-condor-s...me36diffbu~cd/

I was told by a reputable BDR builder that Backdraft removes the 12mm bolt and replaces it with the 14mm bolt when they install these BMW differentials. Is that not the case?

[Soundkillr]

Originally Posted by SBSerpent

I was told by a reputable BDR builder that Backdraft removes the 12mm bolt and replaces it with the 14mm bolt when they install these BMW differentials. Is that not the case?

I can tell you this...I just ordered the bmw brace kit for mine....it comes with a bolt smaller than mine so I would believe bdr does use the 14mm bolt

Mine is sheared as well...and I bought the kit that came with the bolt. So unfortunately I have to find a source for a hardened 14mm bolt...because the one in the kit is too small .

[bobcowan]

Originally Posted by Soundkillr

I can tell you this...I just ordered the bmw brace kit for mine....it comes with a bolt smaller than mine so I would believe bdr does use the 14mm bolt

Mine is sheared as well...and I bought the kit that came with the bolt. So unfortunately I have to find a source for a hardened 14mm bolt...because the one in the kit is too small .

It had a hardened bolt in there to begin with. And that's why it's broken now.

I know I'm going to get a lot of flack for this, but I'll say it anyway. Hardened, grade 8/10.9 bolts are extremely strong along their length, and are great when you need high clamping strength. But in cross section they can be brittle. When subjected to lateral stresses, they will eventually crack and break. Which is what happened to you.

BDR tried to get around that by replacing it with a bigger bolt diameter. Sometimes that works. But here we are.

High grade bolts - 10.9, 12.9 - should only be used for high clamping strength in single shear if there is a locating dowel, or in double shear. Never in single shear in a high vibration environment. .

When I install the support bracket, I will use a grade 8.8. Which is about equivalent to a SAE grade 5. 75,000 psi is plenty in that application.

Carroll Smith once said, "There's no place on a race car for a grade 8 fastener."

[SBSerpent]

Originally Posted by spdbrake

If you're sticking with the stock forward bolt I'd get the improved BMW OEM bolt. https://www.ecstuning.com/b-genuine-...t/33176760337/

Also if you are considering poly bushings and adjustable lower control arms now would be the time to do it. The Diff has to be slid back 2-3 inches and the axles disconnected to remove the upper and lower inboard control arm bolts.

I just reread the last post. "The crazy thing is when I was turning the bolt I thought I saw the opposite end of the bolt turning too". --- Yes broken bolt can be extracted from the rear of the bolt hole without removing the diff.
If you have a Dremel and a cut-off wheel, cut a slot in the protruding tip of the bolt. Then use a screwdriver to walk it out the back of the bolt bore.

Why does the improved bolt have threads near the head of the bolt whereas the sheared bolt that 169mph experienced has no threads near the head????

[Soundkillr]

Originally Posted by SBSerpent

Why does the improved bolt have threads near the head of the bolt whereas the sheared bolt that 169mph experienced has no threads near the head????

The bolt in that link will not work for our cars. The bolt as pictured is a 12mm and we need the larger 14mm

That bolt as pictured is what came in my reinforcement kit and is to small.

[spdbrake]

14mm bolt
https://www.ecstuning.com/b-genuine-...t/33176760336/

14mm upgrade kit with tap/bolt/poly bushing.
https://www.ecstuning.com/b-condor-s...me36diffbu~cd/

14mm upgrade video if you need it.
https://www.youtube.com/watch?v=NHhsjZcfnbk

[169mph]

Originally Posted by spdbrake

14mm bolt
https://www.ecstuning.com/b-genuine-...t/33176760336/

14mm upgrade kit with tap/bolt/poly bushing.
https://www.ecstuning.com/b-condor-s...me36diffbu~cd/

14mm upgrade video if you need it.
https://www.youtube.com/watch?v=NHhsjZcfnbk

Thank you sir. I really appreciate your help!

[169mph]

I have one more question. I assume I unbolted the differential output flanges the part below should have stayed with CV joint. Not sure if theses are spacers I heard others mentioned?? I’m going to have a local shop go through differential just make sure nothing was damaged.

Thanks again for the help.

[spdbrake]

Yup those are the spacers and the way you pulled the axles is correct.

[169mph]

Originally Posted by SBSerpent

I was told by a reputable BDR builder that Backdraft removes the 12mm bolt and replaces it with the 14mm bolt when they install these BMW differentials. Is that not the case?

I can confirm that my differential had a 14mm bolt. I assumed it was 12mm. I stopped at a mom and pop hardware store on my way home today to see if they sold 14mm bolts. Most of the automotive stores only carried 12mm. I got lucky. I planned on tapping the differential tonight and ordering a new bushing but it looks like all I need is new bushing. Regardless I plan on replacing all the differential bushings and replace the differential cover gasket.

[eschaider]

A socket head cap screw is usually class 14.9 in metric talk with a tensile strength equal to or greater than 180,000 psi. This is well north of an SAE grade 8 fastener.

If it is stainless then it is likely at 70,000 psi tensile or lower and probably a Metric class 5.8 (or lower) fastener which is comparable to a grade 3 or 5 SAE fastener and does not belong where you are putting it.

If you use a metric hex head fastener from your local auto parts or hardware store you are likely down in the class three or five category unless you can find a fastener that is marked as class 12.9. If it's head is marked as class 12.9 then it is approximately the equivalent of an SAE grade 8 fastener with 170,000 psi tensile strength. These are not typically in your local retail store.

For safety and longevity reasons you should be using a Metric black oxide steel cap screw. These fasteners will minimally be class 12.9 and possibly class 14.9. The hex head fastener in your picture is a dangerous choice.

When you select your replacement fasteners pick a fastener that has unthreaded shank in the load bearing locations and tightening threads outside of the load bearing locations. You will likely need to use hardened washers to properly position and tighten the fastener.

While I don't know with certainty, I strongly suspect the reason for your fastener failure was the threads on the broken fastener were placed in shear. This is a big no-no. Use the unthreaded portion of the fastener for load bearing purposes like shear and the threaded portion of the fastener for applying tensile load to tension and lock the fastener in place.

You can find the fastener(s) you need at McMaster-Carr or Belmetric.

[169mph]

I’m replacing the bushing too. The kit comes with a 10.9 14mm 1.5 133 hex head bolt. Guess I need to do some research before buying a replacement. Thanks