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Old 01-28-2019, 06:56 AM   #1
zeedre
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Weird problem with diff swap

Hi, I'm doing a diff swap from the original 4.10 to a medium case 3.73 and having a weird problem.

First, the 3.73 holes match up to the mounting bolts fine. However, the big bushing is a full half an inch off set to the mounting flanges with the diff being too high. Additionally, the drive shaft is binding either on the bottom of the car or inside the hole of the floating cross member from above.

So, the entire diff is at least .5" too high - enough to be a problem. Googling yields no similar stories. Anybody ever see this?

I'm thinking my cross beam must be bent but thats a lot of bendage! Should I attempt to put washers above the diff to lower it? Half an inch of washers is a lot but probably would work. What should I do?
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Old 01-28-2019, 08:22 AM   #2
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Do you know if the rear subframe bushing have ever been replaced?

The reason I ask is that vendors like Garagistic sell these "modified" rear subframe bushing to help correct wheel alignment on lowered e30's. As stated on Garagistic's site:

"These are the 'raised height' bushings. That means that these e30 rear subframe bushings will actually raise up the subframe 12mm to compensate for unwanted rear camber."

https://store.garagistic.com/bmw-per...frame-bushings

With this Garagistic subframe bushing kit, since the complete rear drivetrain also needs to be lowered, you would also install (a) four longer studs and 13mm spacers to lower your diff, hopefully an LSD (choose "spacer" option here https://store.garagistic.com/BMW-dif...t-e30-e28-z3); and (b) an offset LSD bushing to compensate (https://store.garagistic.com/bmw-per...0-diff-bushing)

Not saying you have these "modified" rear subframe mounts, but they lower the subframe 12-13mm, or 1/2" that your also off. This would require the PO to have negligently forgotten to simultaneously lower the diff/LSD via (a) 4 bolt mounts; and (b) bushing, but something easy to check (just look at the rear subframe bushings). Like "What are you, dumb?"

If not, without seeing your e30, I would agree something is probably "bent."
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Old 01-28-2019, 08:48 AM   #3
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What car do you have? the ix has a different diff bushing and subframe position - it differs by about 1/2"...
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Old 01-28-2019, 08:54 AM   #4
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What car do you have? the ix has a different diff bushing and subframe position - it differs by about 1/2"...
Since iX never came with a 3.73 ratio IDK (3.91 or 3.64 viscous), but if he's into performance lets hope he didn't replace a clutch type with a viscous!
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Old 01-28-2019, 08:59 AM   #5
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actually, it did come with the 3.73 ratio - it's not common but it exists.
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Old 01-28-2019, 09:02 AM   #6
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actually, it did come with the 3.73 ratio - it's not common but it exists.
Chris, was the 3.73 viscous gearset factory or just tuner's like Alpina, Hartge, Prodrive, etc, swaping gearsets?
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Sucks for everyone involved. Lots of guys are just building IG cars these days... Look good on the internet, but in real life they're hacked up piles of shit.
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Last edited by nogapersnBMWs; 01-28-2019 at 09:27 AM.
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Old 01-28-2019, 09:03 AM   #7
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its a 318i - I've replaced the subframe bushings myself with oem. This wouldnt explain why it worked with the 410 but not the 373 I think.

This ones an open 3.73. Maybe its a bit bent such that the 410 barely fit whilts the 373 which is a bit longer interferes. I do have a clunk that I havent been able to diagnose.

Ok, well, if the garagistic kit has a spacer option, I'll try the washer solution to relocate the diff.
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Old 01-28-2019, 09:08 AM   #8
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Could the bushing on the 4.10 just be worn enough that it still works despite a potentially bent subframe? or maybe the 3.73 has the wrong bushing, or it's installed incorrectly?

does it fit if you swap the diff covers?

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Chris, was the 3.73 viscous gearset factory or just tuner's swaping gearsets?
It's factory with VC; early euro models with automatics.
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Old 01-28-2019, 10:14 AM   #9
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The 410 is a small diff vs the 3.73 is a medium - so different diff covers.

The 410 also had a recent garagistic one, the 3.73 looks older but old doesnt account for .5" off. Both bushings are fairly centered vertically. Also, the ix bushing appears to have the hole higher which is the opposite of my situation. Hmm.
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Old 02-04-2019, 07:14 PM   #10
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Thought

If you are replacing a small case 4.10 with a med case diff the first thing to do is collapse the driveshaft to allow space for the longer nose of the medium case diff. (I think its about 3” longer than the sml case).

Second thought - I think some of the bushings appear to be offset with an orientation mark. (for sure teh M3 bushing hole is not centrally located)
For example the basic one with the solid triangular central casting looks to require installation with the triangle tip pointing upwards. If it’s been installed upside down you might find the hole is 0.5” to high to line up with the holes in the chassis leg bracket.

Some picture might help. Good luck with it.

Last edited by Hsquad12; 02-05-2019 at 05:33 AM.
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Old 03-16-2019, 09:34 PM   #11
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if the hole is .5'' too high maybe unbolting the shocks might drop/tilt the subframe some if u havn't already.
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