M15 M20 M21 M22 M20 XU-1 M21 L34 M21 L34 Muncie
LJ XU-1 XU-1
LH L34 L34
1st 2.48:1 3.05:1 2.54:1 3.74:1 2.54:1 2.54:1 2.32:1 2.52:1
2nd 1.68:1 2.19:1 1.83:1 2.68:1 1.83:1 1.83:1 1.65:1 1.88:1
3rd 1.00:1 1.51:1 1.38:1 1.68:1 1.25:1 1.25:1 1.25:1 1.46:1
4th 1.00:1 1.00:1 1.00:1 1.00:1 1.00:1 1.00:1 1.00:1
Reverse 3.59:1 3.05:1 2.54:1 3.74:1 2.54:1 2.54:1 2.54:1 2.59:1
Cluster gear 2813496 2813487 2820694 9939484 2823899 9939484 3905463
Notes found on the Internet by “Dr Terry”
Hi Guys.
I thought it’s time to put in my 2c worth on the topic of Aussie 4-speeds.
Firstly, the use of the terms M20 & M21 & so on, is very confusing. The 1 code number can refer to more than 1 type of box, so it’s better not to rely on them. For instance an M20 is also an Opel 4-speed, an M21 is a Saginaw, the M22 is another Saginaw & M22 is also what GM in the US call their Hi-Torque Muncie “Rock-Crusher” box. The box in the XU-1 is called an M20 even though its ratios are closer to what we normally call an M21. See what I mean!!
Anyway there are 5 different ratio sets available in the Aussie 4-speed. They are:-
3.05/2.19/1.51/1.00 (Cluster Part No. 2813496) This is the most common version (normally called the M20) & is available with both 6 & V8 front shaft lengths.
2.54/1.83/1.38/1.00 (Cluster Part No. 2813497) This is probably the 2nd most common box & is standard fitment behind 308/5.0 V8s (normally called the M21) & is usually only seen with a V8 length front shaft, although there was a 6-cylinder version homologated as a alternate choice of ratios for the LJ XU-1.
3.74/2.68/1.68/1.00 (Cluster Part No. 2820694) This is the “wide ratio” box (normally called the M22) that is usually only seen in 6-cylinder HQ-WB One-Tonners. It was only ever factory built with a 6-cylinder length front shaft, but since the clutch gear is the same as an M15 3-speed box, you could use an M15 V8 front shaft & make a V8 M22 if you really needed one.
2.54/1.83/1.25/1.00 (Cluster Part No. 2823899) This is the box that is often wrongly referred to as the 6-cylinder M21, when it was actually called the XU-1 M20 (by GM-H). It was standard issue in XU-1s (with a 6-cylinder front shaft, obviously) but the same ratio set was also homologated as an alternate unit for racing for the L34 (with a V8 length front shaft).
2.32/1.65/1.25/1.00 (Cluster Part No. 9939484) This is probably the rarest box of all, as it was only seen as an homologated alternate unit for the LJ XU-1 & the LH L34, with their respective 6-cylinder & V8 length front shafts.
Another common myth to dispel is that there is no difference in strength between any of these boxes, they all use the same bearings & casing etc. A V8 M21 is no stronger than a 6-cylinder M20. The M22 is not stronger just because it�s a One Tonner Box. Also, as mentioned in an earlier post using the ID grooves on the front shaft is not reliable.
Hope this adds to the confusion.
AFAIK the XU-1 M20 was fitted to all CK XU-1s in the LC as well as being standard in the LJ XU-1s. The Opel M20 was fitted to all GTR & XU-1s prior to July ’71.
Dr Terry.