19 replies to this topic

[swig_man]

Hi all

Was chatting with a mate earlier today and the topic came up.....whats the difference between M20 and M21 Gearboxes, and i guess, which cars were they used in
Any info would be great

Thanks

swig

[Collo]

Ratio's, basically...

[Redslur]

An M21 has a taller first and third gear than the M20. M21 was predominantly behind a 308 and M20 behindA 253.

[76lxhatch]

I beg to differ, the six cylinder ones are slightly stronger than the V8 ones because the input shaft is shorter and doesn't put as much load on the front bearing!

[rodomo]

I'd rather blow up 10 aussies than have a Jap box.................

But that's just me.

[swig_man]

Thanks for all that.

But got another question......., some of you guys mentioned ratios.....are there numbers on the gears or shafts that identify M20 or M21??......can you tell from looking from the outside??? are the cases the same??

Cheers

Swig

[76lxhatch]

are there numbers on the gears or shafts that identify M20 or M21??......can you tell from looking from the outside??? are the cases the same??

Cases are identical, can't tell from outside (other than the V8 input shaft grooves as above). Gearsets do have some numbers, I think they were posted here somewhere but searching is difficult these days

the input shaft on the m20 will have one machined ring around the spline
the m21 has 2 machined rings around spline

But only for V8s, and beware there is an M22 ratio set (one tonner, short first gear) in the six cylinder boxes also

[rodomo]

Unknown author? Either Dr Terry or Yel327 I think? Off the forum though:

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).

2.54/1.65/1.25/1.00 (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.

Edited by rodomo, 21 November 2009 - 08:27 PM.

[swig_man]

Your all a bunch of legends

thanks heaps

swig

[yel327]

Rodomo, I'm pretty sure that was one of my posts.

The thing not mentioned in it though is that the Aussie M21 lists a different part number for reverse gear to the other boxes. The gears do interchange so I'm not sure what is different about it. All I can think of is that maybe the Aussie M21 reverse is made from stronger material, hence maybe why GM-H called it the "heavy duty 4spd" or whatever it was. Yet another reason why ANY XU-1 box is NOT an M21, as 3rd gear for the XU-1 is the same part number as M20 and M22 boxes.

Never rely upon rings to show what a box is. Some aftermarket clutch gears have wrong rings, plus for example:

You can stick a V8 M21 input shaft into an M20 box with an XU-1 cluster. Doesn't make it an M21.
You can put a V8 3spd input shaft into an M22 box and make a V8 M22. Rings tell you nothing.

Craig, Opel 4cyl engine in LH with Aussie M20. Input shaft is the same as 6cyl M20, just the bearing dust cover is much shorter.

Note also cases are different depending upon when the box was made. Early boxes use 3spd cases and hence you can't use a later reverse idler shaft, you need a 2 piece shaft. ADR27A boxes had a different side cover to accommodate the switch for 4th gear vacuum advance (switch drives the vacuum switch on teh engine that controls vacuum, via the thermal switch on the head).

[TerrA LX]

Excellent information Yel427, all the more reason these boxes should be scraped altogether and replaced with jap boxes.

[yel327]

Excellent information Yel427, all the more reason these boxes should be scraped altogether and replaced with jap boxes.

NO!!!! Scrapped an replaced with proper M21 and M22 boxes, ie brand new USA made Muncies!

Yes, the old Warwick Yellow GTS327 will be a GTS427, and I suppose when that eventually happens my username will have to change to Yel427!

[76lxhatch]

ADR27A boxes had a different side cover to accommodate the switch for 4th gear vacuum advance (switch drives the vacuum switch on teh engine that controls vacuum, via the thermal switch on the head).

I always wondered what that thing was