My E24 was purchased as a chassis. The E24 is usually fitted with the so-called medium case (188mm) diff. Mine came with a 210mm, but that wasn’t part of the purchase agreement.
I was set on a relatively rare locking differential. Not many people are aware that a few of the late-model Z3 six-cyls have a torsen locking diff that can be made to fit an E24. Unbelievably, after several months of searching, one turned up in my backyard. The donor was an unfortunate Z3 2.2, rear-ended and being parted out. I took a chance on the diff – not all of them are LSDs – and they aren’t labeled. Much to my relief it ended up being just the part I was searching for. I just installed it today, after swapping the cover, stub shafts and speed sensor.