Small case -> medium case. What the diff?!

[rhE-30]

A little background first. My car is a '85 318i, I swaped in a 24 valve engine and getrag 250 (i know it sucks but it's what I got for now). So from some research i learnt that the stock ratio is 3.64:1, and from experience it sucks on the highway. So I went to the junkyard and pulled a diff out of a 325e, it's a 2.79:1 medium case. I swapped the input flanges as that is what i thought was unnecessary in order to mount it up but i guess it's not, the only bolt i can put in is the one from the rubber mount the other four that attach to the sub-frame are off by about 1/2 inch off forward. The drive shaft boltts up.

Am I doing something wrong? Do i have to use a small case? Mounting procedure?

Here are some pics of the diffs.

medium


small

[mr.vang]

yes you will have to use a small case.

[rhE-30]

sub-frame swap?

[mr.vang]

sub-frame swap?

you can, don't forget all new bushing. while your at it, rear disc brake swap.

[MR 325]

Please DO NOT listen to mr.vang, he is 100% wrong.

The medium case is a DIRECT swap in to your car, you do not need to change the subframe. You want to use the flanges that were originally installed on the medium case diff, not the ones from your small case.

[mr.vang]

Please DO NOT listen to mr.vang, he is 100% wrong.

The medium case is a DIRECT swap in to your car, you do not need to change the subframe. You want to use the flanges that were originally installed on the medium case diff, not the ones from your small case.

yes, please discard my misinformation. early this month i did a measurement on both small and medium case, somehow i got different numbers. i just double check and yes mr 325 is right.

[Holland]

Dude, I have an 85 318i, and I swapped a 3.73 medium case LSD from a 325is. I swapped the entire cases directly, didn't change flanges or anything. And actually I had the thing in and out about 4 times already, and every time it swapped right in.

[Bearmw]

I ran a 3.64 small diff in an 87 eta with an i motor swap with no issue.

[SmokeE30]

bringing this thread back from the dead because i just had the same issue on my 88 325. i tried to install a small case 4.10 out of a 91 318i only to realize the mounting points are off by about 3/8" and could not be bolted in.

now the 4 bolt holes on the case are the same as on the medium case but the distance between them and the rear hanger on the diff cover is 3/8" off and thus wont line up.

now i suppose i could bend the rear hanger to make it work but id rather not make a medium case not fit again. everything i have heard says it should bolt right in, but for some reason it does not, so idk what the difference between the small case diffs OP and i got and the ones everybody else fits but it defiantly would not "drop right in"

[kronus]

There might be a difference between early and late small-case diffs?

[Wanganstyle]

I have a hot tank cleaned media blasted early small case 1985 unit I could measure up against a 188mm 2003 z3 case if wanted to know the compatibility between earliest 168mm e30 diff and latest e30 188mm compatible units from z3.

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[CHIF8008]

swapped a small case out for a med. in Nicks car. direct swap. no issues. (early model 325 BTW)

[NEOe30]

bringing this thread back from the dead because i just had the same issue on my 88 325. i tried to install a small case 4.10 out of a 91 318i only to realize the mounting points are off by about 3/8" and could not be bolted in.

now the 4 bolt holes on the case are the same as on the medium case but the distance between them and the rear hanger on the diff cover is 3/8" off and thus wont line up.

now i suppose i could bend the rear hanger to make it work but id rather not make a medium case not fit again. everything i have heard says it should bolt right in, but for some reason it does not, so idk what the difference between the small case diffs OP and i got and the ones everybody else fits but it defiantly would not "drop right in"

Please DO NOT listen to mr.vang, he is 100% wrong.

The medium case is a DIRECT swap in to your car, you do not need to change the subframe. You want to use the flanges that were originally installed on the medium case diff, not the ones from your small case.

Major thread revive and similar question, sounds like you guys have some information i could use. I have a late model 318 with a small case and Im looking to bolt in a medium case from a 325e. Is this a direct swap as well? sounds like there may be fitment issues on the late model?

[Steve Corbett]

No issue. Just swapped out a small case on a 91 318 to a medium case LSD. Biggest issue is getting all 4 bolts to line up. Just get one started before starting the next one. Do not tighten down till all 4 are started. The other option is to convert to studs. Easy alignment then.

[e30sh]

You will have to change the length of the driveshaft slightly as the nose lengths between the diffs are slightly different.