M20 and M21 Gearbox Catalog
Click on the various images to see an exploded view of the componets and parts you are looking for
M20 and M21 Gearboxes:
M20 = 1 groove M21 = 2 grooves M22 = Bare.
Unfortunately the grooves are not a 100% accurate way of IDing the boxs anymore, because some of the repo input shafts available now do not have the grooves on them. I have copied part of this from a post I did a couple of weeks ago.
From memory if the cluster number ends in
496 it is a M20 (3.05:1 low gear ratio)
497 it is a M21 (2.54:1 low gear ratio)
694 it is a M22 (3.74:1 low gear ratio)
If a 6cyl input shafts end in
923 it is a M20 (3.05:1 low gear ratio)
921 it is a M22 (3.74:1 low gear ratio)
If a V8 input shafts end in
925 its a M20
924 its a M21.
If third gear ends in
491 its a M20
492 its a M21
696 its a M22