The information on this page has been compiled from many subscribers of the BMW E21 Digest. Thanks to all of you who helped, especially Sean Woodward!
The time has come - you need a transmission for your six cylinder E21. Maybe the stock 5 speed in your 323i has just worn out. Or perhaps you want to replace the 4 speed in your 320/6 with a 5 speed overdrive. Or maybe you're preparing to swap a 2.5 liter E30 engine into your 320i. For whatever reason, you need a 5 speed transmission that will bolt up to an M20 small six. And you've found that those transmissions are not easy to find, and rebuilding a worn one is rather expensive. What to do?
There is an alternative to those hard to find six cylinder E21 gearboxes: use the five speed from a 528e or E30 325e/i. The ZF transmissions in those cars are easy to find (translate as more affordable), often have lower miles, they will bolt up to our small six engines, and the gear ratios are almost identical to the E21 overdrive box. But there are some issues to deal with, which is the point of this article. Let's look at those issues one at a time.
- The E30 trannies don't have a speedometer drive on them. To have a working speedo when you're done, you need to use an electric speedo from another model (I've been told the early 528e or 325e units fit well, but need to be glued in place) with a "chopper wheel" kit to provide the electric signal for it. (Metric Mechanic has supplied the chopper wheel kits for people I've heard from.) I am not sure how accurate these setups will be, as I don't know how the later cars with electric speedos compensate for the rear axle ratios. It may be that speedometers from some models are more accurate than others, or maybe the chopper wheels are different. Can any one enlighten me?
- The E30 transmission has a pair of rubber mounts at the rear rather than the single one of the E21. The solution to this one is simple - use the rear transmission crossmember from the E30 that goes with the transmission. You will need to slightly enlarge the mounting holes in the crossmember to bolt it up.
- Shifter - use the shift plate that came with the E30 transmission. Assuming you are replacing a worn E21 5 speed, use the E21 shift lever as it gives a shorter throw. You can use the E30 lever if you don't have the E21 lever.
- Driveshaft - The E30 transmission is a tad shorter than the E21 unit. This means the E21 driveshaft is too short. Or is it? There are multiple options for this one.
- Option one - install your six cylinder E21 5 speed driveshaft, and loosen the 4 bolts that attach the differential to the rear crossmember. Place a jack under the middle of the crossmember to take a tiny bit of sag out of it, then pry the differential forward about 1/2" - 5/8". Then tighten it up.
- Another choice is to use the slip-yoke driveshaft out of an '84 E30 318i. This driveshaft's center bearing mounts are in a different location though, and a bit of welding on the underside of the car will be required.
- If you have a six cylinder E21 four speed driveshaft and the flex disc mounts are the same (6 hole vs. 8 hole), you can have a driveshaft shop shorten it by 3.1 inches. This could be done with four cylinder E21 shafts also, but I don't know how much shorter it would need to be.
These tips should get you on the road with your new E30 5 speed. Thanks again to all who sent this information to me, and if anyone has other tips or corrections to offer, feel free to send them to me.