| Notes found on the Internet by “Dr Terry” |
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| Hi Guys. |
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| I thought it’s time to put in my 2c worth on the topic of Aussie 4-speeds. |
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| 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!! |
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| 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. |
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| 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. |
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| 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. |
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| Hope this adds to the confusion. |
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| 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. |
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| Dr Terry. |
