Finally completed my diff build! It has 3.73 gears, a ZF 2 clutch 45/45 diff converted to 4 clutches using a Racing Diffs setup. The R&P, bearings and races are REM poished. Carrier preload is 10 in/lbs, pinion preload w/ seal is 16 in/lbs and the backlash is a hair over .005". Gear pattern on drive side looks good...forgot to take a pic of the coast side of the gear.
The story...I sourced a diff from an E24 633CSi which came with a large case (210mm) diff with a ZF limited slip unit and 3.73 gears. Ideally I would have had 3.85 gears. I plan to run smaller diameter tires and a 3.91 will be a tad shorter forcing a couple of more shifts per lap. Also stayed with 45/45 ramp angles for now. Ideal setup (I think) will be a 3.85 with 30/60 ramps. But before I sink more money into this...let's see if I'm capable of building a diff!
While this wasn't easy, so far it hasn't been super complex. The biggest issue is the lack of info on BMW diffs which are a little different than most others. Mostly in how you measure carrier preload. I figured out that you set carrier preload first. Then shims, I'm pretty sure the shims are specific to the case with maybe some minor adjustments for a specific R&P. The R&P has markings on them...have no idea what they mean but I'm sure it has something to do with calculating the ideal shim for pinion depth. I reused the same shims that came with the case (NOT the 3.73 R&P) and the specs came out perfect without adjustment. My theory (not saying its fact) is that the shim that comes with the case is a great starting point. The ideal shims will be close to what came with the case.
If the shims come out right the first time, then life is good. The next hurdle is pinion preload. It takes awhile to get preload and then it goes from 0 to 30 in/lbs in about 1/16 of a turn on the pinion nut. I ruined 4 crush sleeves and 2 pinion seals figuring this out. Finally learned how to press the pinion in without ruining the seal. I used an air impact and greased the pinion nut. I used longer blasts to take the play out of the input flange. As soon as I got some preload, it was a short blast of the impact, remove flange holder (bolted down to flange), check preload, install flange holder, another blast and repeat). It takes patience. I know BMW has a special tool to hold the flange, trying to figure this out. This would make like much better instead of bolting and unbolting my flange holder (4' long cheater bar with holes) 10-15 times)
Overall, its not the impossible task. You don't NEED a press but it makes it go MUCH, MUCH quicker.
Special Tools Needed:
BAP - big ass press or could use bearing separator but this is MUCH easier!
BFH & punch
Bearing separator
Race/seal driver
32mm socket
Dial indicator w/ base
Input flange holder (fab'd from 4' long 1/8" steel L-bar)
Adaptor for output flange to measure carrier preload (steel bar, 2 holes drilled to bolt to flange, hole on end with bolt to measure preload)
Beam style torque wrench (in/lbs) for preload
1/4" female to 3/8" male socket adaptor (to fit 32mm socket to beam torque wrench)
3/8" female to 1/2" male socket adaptor (to fit 32mm socket to beam torque wrench)
Gear marking compound (I used diaper rash cream...it worked ok)
Steps to assemble the diff
Step 1 - Install LSD unit with bearings into case (no pinion gear installed)
Step 2 - Set carrier preload & remove LSD unit
Step 3 - Install pinion gear races into diff case
Step 4 - Install pinion gear and input flange - do not crush sleeve or install seal yet! - & install
Step 5 - Install LSD carrier
Step 6 - Check gear pattern and adjust (may need to change pinion shim and and/or carrier shims)
Step 7 - Check backlash
Step 8 - Remove LSD unit & pinion gear
Step 9 - Install crush sleeve on pinion gear
Step 10 - Press pinion gear into case
Step 11 - Install input seal
Step 12 - Set pinion preload (big impact - tighten nut to take up/down play out of input flange, tighten nut with short blasts if the imact, as you get closer to preload spec - use shorter blasts)
Step 13 - Reassemble and re-check gear pattern and backlash - should not need any adjustment
Don't forget the carrier cap O-rings!
Some pics...
The LSD unit being assembled...
The carrier ready for some LSD goodness!
The carrier races being installed
The LSD unit is together
Gear pattern on the drive side looks good!
Proof of backlash...lol
The story...I sourced a diff from an E24 633CSi which came with a large case (210mm) diff with a ZF limited slip unit and 3.73 gears. Ideally I would have had 3.85 gears. I plan to run smaller diameter tires and a 3.91 will be a tad shorter forcing a couple of more shifts per lap. Also stayed with 45/45 ramp angles for now. Ideal setup (I think) will be a 3.85 with 30/60 ramps. But before I sink more money into this...let's see if I'm capable of building a diff!
While this wasn't easy, so far it hasn't been super complex. The biggest issue is the lack of info on BMW diffs which are a little different than most others. Mostly in how you measure carrier preload. I figured out that you set carrier preload first. Then shims, I'm pretty sure the shims are specific to the case with maybe some minor adjustments for a specific R&P. The R&P has markings on them...have no idea what they mean but I'm sure it has something to do with calculating the ideal shim for pinion depth. I reused the same shims that came with the case (NOT the 3.73 R&P) and the specs came out perfect without adjustment. My theory (not saying its fact) is that the shim that comes with the case is a great starting point. The ideal shims will be close to what came with the case.
If the shims come out right the first time, then life is good. The next hurdle is pinion preload. It takes awhile to get preload and then it goes from 0 to 30 in/lbs in about 1/16 of a turn on the pinion nut. I ruined 4 crush sleeves and 2 pinion seals figuring this out. Finally learned how to press the pinion in without ruining the seal. I used an air impact and greased the pinion nut. I used longer blasts to take the play out of the input flange. As soon as I got some preload, it was a short blast of the impact, remove flange holder (bolted down to flange), check preload, install flange holder, another blast and repeat). It takes patience. I know BMW has a special tool to hold the flange, trying to figure this out. This would make like much better instead of bolting and unbolting my flange holder (4' long cheater bar with holes) 10-15 times)
Overall, its not the impossible task. You don't NEED a press but it makes it go MUCH, MUCH quicker.
Special Tools Needed:
BAP - big ass press or could use bearing separator but this is MUCH easier!
BFH & punch
Bearing separator
Race/seal driver
32mm socket
Dial indicator w/ base
Input flange holder (fab'd from 4' long 1/8" steel L-bar)
Adaptor for output flange to measure carrier preload (steel bar, 2 holes drilled to bolt to flange, hole on end with bolt to measure preload)
Beam style torque wrench (in/lbs) for preload
1/4" female to 3/8" male socket adaptor (to fit 32mm socket to beam torque wrench)
3/8" female to 1/2" male socket adaptor (to fit 32mm socket to beam torque wrench)
Gear marking compound (I used diaper rash cream...it worked ok)
Steps to assemble the diff
Step 1 - Install LSD unit with bearings into case (no pinion gear installed)
Step 2 - Set carrier preload & remove LSD unit
Step 3 - Install pinion gear races into diff case
Step 4 - Install pinion gear and input flange - do not crush sleeve or install seal yet! - & install
Step 5 - Install LSD carrier
Step 6 - Check gear pattern and adjust (may need to change pinion shim and and/or carrier shims)
Step 7 - Check backlash
Step 8 - Remove LSD unit & pinion gear
Step 9 - Install crush sleeve on pinion gear
Step 10 - Press pinion gear into case
Step 11 - Install input seal
Step 12 - Set pinion preload (big impact - tighten nut to take up/down play out of input flange, tighten nut with short blasts if the imact, as you get closer to preload spec - use shorter blasts)
Step 13 - Reassemble and re-check gear pattern and backlash - should not need any adjustment
Don't forget the carrier cap O-rings!
Some pics...
The LSD unit being assembled...
The carrier ready for some LSD goodness!
The carrier races being installed
The LSD unit is together
Gear pattern on the drive side looks good!
Proof of backlash...lol
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