Manual Gearbox and Differential Options
 

Mk1 S12 - Uses a "B" type gearbox (FS5W71B). Shorter than the later C type and therefore much shorter than an S13/14 C type. You can use the S12 C type in a Mk1 but it needs a custom mount and at least the front half of the C type prop mated to the rear half of the B type prop.

Mk2 S12 - Uses a "C" type gearbox (FS5W71C). This is essentially the same gearbox used in the S13, S14, Z31 N/A, R33 GTS and possibly others. However, in the Mk2 S12, the tail section is shorter. You can fit a longer C type to a Mk2 S12, mixing the prop shafts as above but the gear selector will sit further back so floor cutting and interior trim fettling is required. The manual and auto Mk2 S12 boxes are interchangeable although the auto cars had a different diff ratio.


Bellhousings - both the B type and all the C types should have the same gearbox to bellhousing bolt pattern. CA18ET gearboxes with bolt directly to CA18DET engines but the SR20DET will require a bellhousing swap if you want to keep the S12 B or C gearbox.

Clutches - The B type and the C types all share the same input shaft spline diameter and tooth count (25.6mm x 24T). The CA18ET crank is 6-bolt, the CA18DET crank is 8-bolt so you cannot use an S13 flywheel on an S12 engine but the S13 clutch will fit on the S12 flywheel and should be able to hold as much torque as most people could make from an ET.

Differentials - both the Mk1 and Mk2 use the same long nose R200 diff housing. The diff itself is open. Any LSD designed for the S13 open diff should drop straight in (Kaaz,Quaife,OBX). The S13 VLSD will fit but it requires some fettling:



Work in progress, if anything is incorrect, let me know!