Here’s the story.

Any E28 rear differential will fit, any E30 differential will fit. The reason for swapping the rear differential is to attain a limited slip, which happens to be a clutch pack type. Many E30s came with the diff, check the tag on the case to see if you have one. You can also jack the car and spin the wheels. If they turn the same way, then you have a limited slip.

The E30 M3 is fabled to have the strongest 4:10 you can get onto an E30 without major modification. This is true. The case and internals, except for the gearing, are the same on an E30 M3 as on the E28. The lower gear number, the “higher” gearing you will attain. On a 4.10 differential, the driveshaft spins 4.10 times per complete revolution of the rear wheels. (this is an extrapolation)

Now, onto fit. If, like me you were a small case LSD owner (1991 318i) and seek to do an engine swap, you should replace the rear differential. It will not last. Even for a M50 BMW swap which makes just over 200hp. To swap to a larger diff you need nothing if your replacement is from another E30. If the replacement is from an E28 5-series like mine was, you’ll need E30 axle cups (demonstrated below) as well as a rear cover from an E30. I was able to get a great deal on my diff, otherwise I wouldn’t have gone to the trouble. Onto the pictures.

-Alex

Above we have from left, a small case 318i axle snub, a E30 325 snub and finally the removed E28 535is snub. Note the C-clip groove locations (this is why the small case parts don’t fit) as well as the bearing seal diameter and axle bolt circle.

These rear covers are different. The left is the small case 318i, the right is the big case diff, now converted for use on the E30. Note that the small case has a 6 bolt cover and the big case, an 8 bolt cover.

See inset of picture for description.