My $150 diy 4 clutch, 3.73 diff. You can do this too!

[the51Cness]

Since I bought my m3 almost 2 years ago, I've been slowly working toward refreshing the entire car as a daily driver/track/autox dream. I recently broke my front diff bolt and couldn't deal with the dreaded clunk anymore. I hastily replaced the bolt and pulled a quick torque spec on my phone while in the garage. Unfortunately, I accidentally pulled the e46 m3 torque spec and snapped the bolt. Ouch. Please don't judge me. I was able to drill out the first one with a nice cobalt pilot tipped bit. The second one didn't go quite as well (see photo for results). Once again, don't judge me haha



Since I failed miserably at a somewhat trivial task and ruined my diff case, I decided that the next logical step would be to replace that problem with a far more complex and difficult one: pull a couple of junk yard diffs and graft together a performance setup that would otherwise cost me over $1400 at Turner. I already had experience rebuilding an ARB air locker for my Land Rover, so I was somewhat confident that I could figure this out as long as I gave a little more care to it than I did to that poor diff bolt. I began with google and found some great resources on the 4 clutch lsd upgrade, diff construction, as well as which cars came with which gear sets. I will post all links that I used at the bottom of this post. I decided that I wanted either a 3.73 or 3.64 ratio and found that all 188mm or medium bmw gear sets before e46 are interchangeable. This essentially means that all 6 cylinder 3's and 5's before e46 have gears I could use. Awesome. I gathered some tables for which cars came with which ratios and scavenged the local pick n pull. Unsurprisingly, there were a number of e36's, so I found a 325 that already had the transmission and exhaust removed. I went ahead and pulled the diff housing. Next, I got a little lucky and found an 89 525i 5 speed where I really scored since it had a 3.73 LSD gear set. My plan from here was to take the e34 gears, clutch packs from my m3 and place those in the newly acquired housing. Simple right? Four days of work and $80 later I was unable to mash the pinions out without a press, so I drove home with this:



I then set out to work and ordered a new pinion crush sleeve, new seals, 2 quarts 75/140 synthetic lsd fluid, pinion lock ring, diff case gasket, and baby rash cream as marking compound all for about $50. I didn't order new bearings or carrier crush washers because the e34 ones looked decent with minimal pitting, and also because I'm a little lazy, If I wasn't rebuilding my diff now, I couldn't be any worse off anyways. Most importantly, however, I did not want to have to deal with shimming the gear set. I didn't touch any of the alignment related parts from the e34 box, so that preload and backlash theoretically would not change when I moved it to the e36 housing. This included the carrier bearing caps and all shims since shim sets cost more than this project cost me altogether without mentioning that they are very get a hold of. I then took the liberty of using my university's machine shop (I'm studying mechanical engineering) to machine the carrier cap and set my pinion preload. I measured the clutch packs and dog plates to be about 4.2mm together, so I machined that amount off the carrier cap in addition to removing the 4mm spacer in the carrier.





(continued on next post)

[the51Cness]

I was happy with that finish, so proceeded to assemble the carrier. I took special attention to the clutch disks and dog ear plates since they were all used. I tossed the more worn out clutches toward the middle and pointed the 2 most worn dog plate faces toward the belleville washers. The nice part about having 2 diffs on hand is that I was able to easily replace the oiling rings after accidentally breaking one set upon reassembly. I also made sure to paint the clutches with lsd fluid upon install.



I then assembled the diff completely with the old pinion crush sleeve to check gear mesh quality. As I had planned, reusing the shims and crush sleeves left me with perfect alignment. One of my concerns with this build, however, was the ring gear from the older car. Pre-e36 cars came with different sized ring gear bolts which require a set of spacers to fit properly on an e36 carrier. My original intention was to swap over the gears and clutch packs to my m3's carrier, but after some thought, I decided that it was more logical to use all of the e34 parts and just add my m3 clutch packs since all the parts carry over fine. Backlash came out to ~0.003" and the gear mesh marks looked good to me as well. I was pretty stoked to nail this on the first try!



Next, I moved to preloading the pinion bearing since it has no torque spec like the carrier bearing caps do. This means that preload is not preserved upon reassembly. It took me a couple of tries for this, so make sure you buy a couple of crush sleeves if you decide to tackle this yourself. Using an impact wrench and a ghetto rig assembly consisting of a small electronic scale, a ratchet, and a spacer, my preload came in at around 16 inch pounds. I didn't have access to an inch pound wrench and didn't feel like dropping $40 for something I would never use again, so I came up with this crude way of measuring torque applied to the pinion. Once again, please don't judge me haha - I don't recommend this at all. It's a terrible idea.



After that, I reassembled the box, measured the same backlash, and went on my way to toss it in the car. I guesstimated and added a little less than 2 quarts of fluid before putting the cover on because I didn't feel like figuring out a way to pull the plugs. Got my torque specs right for the diff bolt this time, and off to the freeway I went. I noticed that this box whines a little more than my last one which is noticeable since I have no sound deadening or interior in the back of my car. I also noticed a bit of chattering under slow, tight cornering from the lsd unit which I hear is pretty normal. I've driven about 500 miles and it hasn't gone away, so I'm considering getting some lsd friction modifier even though I used Valvoline lsd specific gear oil.

Massive thanks for user: orangehatter for his diff thread - I learned quite a bit and would not have succeeded at this without his post. In the end, with the proper tools, some patience, and quite a bit of reading, this wasn't incredibly difficult and I think that anyone with some decent DIY experience can handle this. Just be very cautious through the entire process as your mileage may vary. I would also like to add that I have been seeing a near negligible drop in highway gas mileage according to the dash display in my car. I have also noticed a very positive increase in acceleration and therefore recommend this mod quite a bit to anyone who doesn't mind a bit more cabin noise on the freeway.

Important resources:

Diff Thread
Bimmerdiffs Rebuild Guides
E36 Ratios
E34 Diffs
E30 Diffs

[M3Dynamic]

whata teaseEdit:Ahh i see what you did there

[TriniSpeC]

Lmao this captured my attention.

[hide1]

So much fail.

[the51Cness]

Yea, I dont know whats going on here. My first post went through, but when I made my second, they both disapeared for no apparent reason...

- - - Updated - - -

There we go. Thanks mods for fixing that

[hide1]

Ooo I can see now

[hide1]

Great job tackling that and sorting through all that information. You'll make a good ME. But get yourself a decent in/LB wrench. I use it all the time.

[NeilM]

Since there's no decent supply of junkyard BMWs where I live, my 3.46 and 3.73 donor diffs came from eBay. Both were E30 parts. Diffs are heavy, so shipping can be a bit of a killer. We didn't rebuild the clutch packs, although I might do that sometime.

For the simplified DIY rebuild, where you don't reset the pinion to ring gear backlash, the key seems to be preserving the exact original shim setup on the E36 diff housing you use. My supposition is that the diff innards are pretty precisely machined, and that whatever dimensional variations there are occur mostly in the case. This makes sense if you consider the size of the case and that the input and output shafts are at right angles to one another, making tolerances harder to hold. It turns out that keeping the case shims allows swapping out the guts without resetting gear backlash. You do tend to get a little more gear whine this way, but nothing serious. A professional diff rebuilder will of course go the extra step of reestablishing correct gear mesh/backlash.

Although I'd also recommend using an in-lb torque wrench for the crush sleeve, an experienced rebuilder (yeah, that's not me!) can gauge the correct level of preload friction on the input shaft by feel.

Neil

[the51Cness]

Agreed. Besides the tolerances between cases, I was also concerned with a new set of bearings changing my backlash since pre load distance would most likely change. I consider myself lucky that everything seems to have come together nicely with the stock shims which is why I warn that your mileage may vary. I think the other key component to the successful swap is the carrier bearing cap. Keeping the shims and caps married is very important since I would expect those to have there own imperfections as well. Moreover, if you op to not change the carrier crush sleeves, it is critical that they be joined with the same bearings and what not. I trusted this decision based on the bimmerdiffs guides I posted where they say it is ok to pull the carrier and then re install without changing shims or crush sleeves.

[Limitedslipm635]

Hi
I would like to ask you about the BMW 188 lsd 4 clutch upgrade. We also have an e36 and I want more lock on the rear wheels . My question is that when machined of the 4.2mm from carrier cap it isn't get too weak? Does it work well since you lowered it? I think lowering the surface could remove the hardened surface of the carrier cap.