Original Author: Jeff Ireland
Differential Types - 2002 and 320i:
Early 1600's and 2002's came with a long neck differential. These were phased out in early 1969. They were either 4.11:1 (1600) or 3.64:1 (2002). They are very rugged (heavy) and easy to come by, but not rebuildable due to the lack of available replacement parts. If you want to switch to the later short-neck differential, you must replace the entire subframe and rear suspension. This sounds tough but is actually fairly easy and the parts are readily available.
All later 2002's and all 320I's used the short neck differential. These came in several ratios:
- 3.36:1 -- 2002 Turbo
- 3.45:1 -- some euro tiis and 6 cyl E21s
- 3.64:1 -- most 2002s and all 4-speed e21s
- 3.90:1 -- some 1976 2002s and all 5-speed e21s
- 4.11:1 -- all 1600s
Of all these variations, all of them can be transplanted into a 2002.
Variations and how to recognize them:
All 2002/1600 diffs have 4 bolts holding the side covers and the output flanges are held in by a large central bolt. CV's are attached the output flanges with 6 - 8mm bolts.
Early e21 ('77 and '78) 320i's have four bolts holding the side covers, but the output flanges are held in by a C-clip inside the diff. These diffs usually have 8mm bolts for the CV joints but some use the later 10mm bolts. These early diffs are the same width as the 2002 diff so no spacers are necessary if you are putting one in a 2002.
Later e21's have 6 bolts holding the side covers and the output flanges are held by a snap ring just above the splines. You can easily pop out the flanges with a screw driver. These diffs nearly all use the 10mm bolts for the CV joints. These diffs are also narrower than the earlier diffs. If you want to put these into a 2002 you should use spacers to bring them to the proper width. Inner CV joints from a 1979 (and only a 1979) e21 are thicker and can be used instead of spacers if you happen to come across a set. Otherwise, billet aluminum spacers and the proper bolts are advised.
Making the Swap:
It's common to put e21 diffs into 2002's either to get the 3.90 ratio or the easily available 320is limited slip. The e21 diffs will bolt right onto the subframe and driveshaft. Just remove the 320i rear cover and install your 2002 cover. Please note the possible difference in widths I mentioned above.
The final issue is 8mm bolts vs. 10mm bolts. If your new diff takes 10mm bolts for the CV joints, you have a couple of choices. Put 320i CV joints on the inboard end of the 2002 axle (if you can find 1979 ones, so much the better), or option two is to drill and tap six new 8mm bolt holes in the e21 output flanges. Clearly these need to be located with some precision and thus we offer the service at Ireland Engineering.