Swapping my diff, questions.

[Cheetah223]

I'm throwing a new (to me) diff into my 84 318i next weekend, getting it resealed right now..It's an open 4:10 that'll be going in, and I can't find information on the stock diff anywhere. I found on one other forum that my car's got a bit different gear ratio, but they didn't say exactly what. Does anyone know what the stock gear ratio on my 84 would be? If it's not 4:10, does that complicate the install of the 4:10 at all, or would it still just drop in? Thanks!

[Dissembler]

I know that my 85 318 has a 4.10 in the rear. All the diffs are interchangeable between all the e30s, but the early 318s used the small diff casing (which are not available with lsds) and all the other models use a large casing. Same mounting points and all that, just different casings. I'm not sure if the input/output flanges are different though

[Cheetah223]

I read somewhere that the large case diffs need the driveshaft to match them, and vice versa with the small case diffs. It's an open, so that at least keeps the odds in favor of being a drop in job. I just ran across something that mentioned like a 3.91 instead of 4.10 so I had to bring the question up..

[Reid]

I read somewhere that the large case diffs need the driveshaft to match them, and vice versa with the small case diffs.

No, the medium case (6 cylinder) and small case (4 cylinder) diffs are completely interchangeable. I just pulled a 4.10LSD (small case) our of my E30 and replaced it with a 3.73LSD (medium case) last night.

[FrantiC]

Yeah they are the same thing, Output and input flanged are identical, mounting holes are identical. Just teh case sizes are different size but that doesn't effect the swap at all, except that its a tougher diff.