Need Differential Rebuild Recommendations

[siamkazi]

This is a dangerous assumption, ask me how I know.

I needed a thicker shim on the RH bearing cap, and a thinner on the LH one, I also needed a thinner shim for the pinion, and by some.

It helps to have shims of different sizes but you won’t know until you are there and mesh it. I guesstimated shims I would need based on loose assumptions and ended up with shims I did not need and still had to get very specific sizes.

if you do it yourself be ready to take your time, if you let someone else do it, make sure they really commit to do it right and not just do a half ass job, because once there I can see why people would call it done because that was what they could do and you end up with whining noises.

Appreciate the feedback, i'm not very well versed in diffs so i posted what Pedro told me, this is very good to know. For future reference for others who come across, what are some signs/symptoms that a diff rebuild didnt go as planned? (i.e. whine/drone etc)

[maupineda]

For shims I used an eBay store that carries a lot of diff parts, I got tons of pinion shims, yet, they did not have the size I needed (because I made assumptions as suggested here), luckily, the size I needed was included in a set I was still able to get from BMW themselves. The eBay shop has a lot of output flanges shims, but again, without being in your diff, you are shooting in the dark, and even then, you will very likely need to machine / lap shims for proper setup.

side cap shims sources

• BMW (see if they have any sets available)
• Racing Diffs - they have shim kits, but is 200+ euros - https://racingdiffs.com/products/bmw-differential-backlash-adjustment-shim-kit?srsltid=AfmBOorNYqH7tAOPtKPEXmmfrz3qz7tpfka7rG VGd8zuYdOsXMVjf_vO
• eBay shop - https://www.ebay.com/sch/i.html?item=165686789013&rt=nc&_trksid=p4429486.m3 561.l161211&_ssn=bmw_original_parts_stock (many pinion and side cap shims available)

Pinion shims

• BMW (see if they have any sets available) - the 3.2 - 3.475 kit is still available (33129065173), there are several other sets , just change the last number to 4/5/6, etc. some are not available anymore.

So, as you see, whom ever does it, needs to get in there, change the gears and mesh it, based on what they have, they need to start trialing different shims UNLESS they have the factory tool and know from the start what shim they need, but even then, they MUST do some back and forth to mesh it properly.

To put in in perspective, to perfectly align the pattern I went to complete different shims sizes than the original, so the assumption that the original shims are close enough is not correct, will it work, yes, but it will leave you with whining noise, unless you win a lottery ticket and it all lines perfectly (SUPER UNLIKELY)

My build details
- pinion from 3.675 to 3.405 (using original BMW shim)
- side RH from 1.475 to 1.330 (I still need to lap the 1.330 shim down to 1.325 to keep the same thickness as OE), the 1.330 shim is from the racing diffs kit.
- side LH from 1.775 to 1.950 (using an unmodified shim from racing diffs shim kit)

Diff rebuild is not difficult, but is very finicky job, super time consuming, AND, if you do it yourself expect it to take days if not weeks of downtime. mostly because once you have it apart and figure out the sims you need, now you need to source them. or buy 10's of sizes, but it gets expensive.

I will say this again, a shop, unless fully dedicated to diff work, with the ability to machine their own shims, is very most certain to do a half ass job because of the level of precision needed, the time it takes, and the lack of shim availability. This is why people like Dan from Diffsonline charge an arm and a leg.

Good luck with your build!

[ThatDudeZac]

This is a dangerous assumption, ask me how I know.

I needed a thicker shim on the RH bearing cap, and a thinner on the LH one, I also needed a thinner shim for the pinion, and by some.

It helps to have shims of different sizes but you won’t know until you are there and mesh it. I guesstimated shims I would need based on loose assumptions and ended up with shims I did not need and still had to get very specific sizes.

if you do it yourself be ready to take your time, if you let someone else do it, make sure they really commit to do it right and not just do a half ass job, because once there I can see why people would call it done because that was what they could do and you end up with whining noises.

Any shim kits you recommend? I'll be having a shop do the job since I don't have the special tools for rebuilding

[ThatDudeZac]

You can source Timken bearings from Amazon - it was about $150 the last time I checked. Probably a little more now. Then you need the seals, o-rings, crush sleeve, and ring gear bolts - $250, 3.91 gear set - about $1100 from the dealer, ZF LSD - $1000 to $1500 used on eBay. Equal length axle shafts - $400/pr new from dealer

So $3.5k total.

Racing Diffs has a 4 clutch conversion for the ZF, works fine. They also have pressure plates with variable ramps machined in...not sure about that. I may give it a try at some point.

I'm in Houston, I have a guy that can build the diff if you supply the core and parts. $400-$500.

3.5k 🤠 Now that's much more what I was expecting to spend! If I can't find a shop local to do the rebuild for a good price I'll definitely hit ya up!

[ThatDudeZac]

Just went through this. I used Peaky parts (Pedro) and had Unitrax here in Corona do the rebuild. 950ish and I supplied the bearings, ring, pinion.

This is great to hear! I'm currently messaging him! I've seen peakyparts recommended 3 times now!

[maupineda]

Just went through this. I used Peaky parts (Pedro) and had Unitrax here in Corona do the rebuild. 950ish and I supplied the bearings, ring, pinion.

This is a dangerous assumption, ask me how I know.

I needed a thicker shim on the RH bearing cap, and a thinner on the LH one, I also needed a thinner shim for the pinion, and by some.

It helps to have shims of different sizes but you won’t know until you are there and mesh it. I guesstimated shims I would need based on loose assumptions and ended up with shims I did not need and still had to get very specific sizes.

if you do it yourself be ready to take your time, if you let someone else do it, make sure they really commit to do it right and not just do a half ass job, because once there I can see why people would call it done because that was what they could do and you end up with whining noises.

[maupineda]

You can source Timken bearings from Amazon - it was about $150 the last time I checked. Probably a little more now. Then you need the seals, o-rings, crush sleeve, and ring gear bolts - $250, 3.91 gear set - about $1100 from the dealer, ZF LSD - $1000 to $1500 used on eBay. Equal length axle shafts - $400/pr new from dealer

So $3.5k total.

Racing Diffs has a 4 clutch conversion for the ZF, works fine. They also have pressure plates with variable ramps machined in...not sure about that. I may give it a try at some point.

I'm in Houston, I have a guy that can build the diff if you supply the core and parts. $400-$500.

Koyos can be found for 180 plus tax for all bearings. Sides are 26 each, large pinion bearing is almost 60 and small one is 50. I got them in eBay.

[siamkazi]

Just went through this. I used Peaky parts (Pedro) and had Unitrax here in Corona do the rebuild. 950ish and I supplied the bearings, ring, pinion.

[0-60motorsports]

Wavetrac is junk, Drexler is by far the best option for these cars.

Are you talking about for turbo cars or NA cars? What's the price difference?

[EthanolTurbo]

F it im getting a wavetrack and then also putting back the 3.62 gears (if i can find a mint set)....and if i cant i'll just stick to my 3.91 gearset thats already in there.

Wavetrac is junk, Drexler is by far the best option for these cars.

[EthanolTurbo]

Following this thread. Im also researching this same topic.
The most recommendations, even from my local shop was to get one from Diffs Online.

I'm leaning towards Diffs online with 3.91 M-Variable Mainly for convivence and piece of mind that the diff was rebuilt well.
Another alternative is to source ring an pinion to desired ratio, and get a rebuild kit from racing diffs (not sure on quality of these) that has clutches, seals and bearings, and finding a shop to do the rebuild, or like mentioned above have them source stuff for you.
you can find the ring and pinion from Peaky Parts, they have some on ebay.

Waste of time. Take it to Pro Gear in San Diego, they do a full rebuild including all parts for under $900.

[ChapterM3]

Following for future reference! Good thread for 2025

[bigjae46]

You can source Timken bearings from Amazon - it was about $150 the last time I checked. Probably a little more now. Then you need the seals, o-rings, crush sleeve, and ring gear bolts - $250, 3.91 gear set - about $1100 from the dealer, ZF LSD - $1000 to $1500 used on eBay. Equal length axle shafts - $400/pr new from dealer

So $3.5k total.

Racing Diffs has a 4 clutch conversion for the ZF, works fine. They also have pressure plates with variable ramps machined in...not sure about that. I may give it a try at some point.

I'm in Houston, I have a guy that can build the diff if you supply the core and parts. $400-$500.

[0-60motorsports]

Want the 3.62 I took off for my 4.1 rebuild?

I'll message you.

[maupineda]

F it im getting a wavetrack and then also putting back the 3.62 gears (if i can find a mint set)....and if i cant i'll just stick to my 3.91 gearset thats already in there.

Want the 3.62 I took off for my 4.1 rebuild?