Gearbox & driveline
Nearly all E21's had the 4-speed manual gearbox fitted. 5-speed boxes are a sought after item for conversion. If you buy a 5-speed try to get it with the matching propshaft (shorter than with 4 sp boxes) and gear shifter meachanism (also shorter). Not all gearboxes fit, E30 boxes lack the built in speedo drive and converting your speedo is not that easy. Better stay away from that. There are both dogleg close ratio 5 speed boxes with a prise direct 1:1 final drive as well as the more common overdrive fivespeed. All gearboxes manufactured by Getrag and M20 6 cil boxes show an M60 sign on the side of the housing.
All E21's were also available as automatic using a ZF 3HP-22 3-speed auto gearbox.
Gearboxes usually last a long time, if you experience clutch judder this might indicate clutch plate is getting thinner. Annoying but usually no need for immediate replacement. Clutch judder is a common issue, especially when the clutch warms up like in a traffic jam, and probably you will mainly experience this when operating the clutch so with clutch slipping, meaning once you let go of the pedal it's ok. Then really the clutch plate is the cause. In some cases it is possible that worn parts in the driveline may worsen the effect, like gearbox mount bushing, propshaft bearing, propshaft centering sleeve, flex disc (guibo) or the diff hanger rubber. A little more remote but also a possible option for wear are the large rear beam bushes at either far end.
Clutch system is hydraulic. Doesn't need adjusting but the system can leak. Info on fixes at the bottom of this page.
Some hints for preventing clutch judder:
NEVER pull away from a standstill in second gear
NEVER hold the car on a hill with the clutch
NEVER let your foot rest on the clutch pedal when the car is moving, even the slightest touch can cause the clutch to slip
ALWAYS slip the clutch as little as possible when pulling away
ALWAYS select neutral and bring up the clutch pedal if you're stationary for more than a few seconds
overview of all gearboxes
5-speed 6 cil M20 boxes compared
There aren't too many general driveline issues. Common wear problems are a torn rubber gearbox crossmember support and a worn propshaft center bearing. Both might cause vibrations while driving or accelerating. As with most parts from BMW, replacement parts are available.
All available diff (final drive) ratios can be found on the specs pages. For an interesting topic on establishing if you have a regular open or a limited slip differential (lsd) the following topic is very useful. It also explains how to establish your diff ratio: http://www.bmwe21.net/forum/viewtopic.p ... 3&p=154898
Replacing clutch plate
Identifying a Limited Slip Diff or LSD
Standard: SAE80, 75W90, 80W90
Uprated: Castrol Syntrans B 75 W
ATF D2 system Dexron
Uprated: Castrol Syntrax 75W-90
Limited Slip Diff REQUIRES special oil! Recommended: Castrol Syntrax Limited Slip 75W-140 (formerly known as SAF-XJ)