43 replies to this topic

[LC-069]

I know this has been done before but
how do you identify a m20 and m21??

Just pulled apart my gearbox and on the cluster gear is has:
HOLDEN 2813498 AP

on the casing 9938391

Thanks guys

[FastEHHolden]

There is an id list in the current street machine....the ratios define what is what.

[Toranavista]

Or simple way by the input shaft. One groove --> M20, Two grooves --> M21. Forget the number of grooves for M22, but think it is two as well, but different ratios.

To unreliable Pat. From personal experience.

[FastEHHolden]

I have the SM article in from of me...They say:

Grooves.
M21 = 1 groove
M20 = 2 grooves

Ratios
M21 (M22 also)
1st 2.20:1
2nd 1.64:1
3rd 1.24:1
4th 1:1

M20
1st 2.52:1
2nd 1.89:1
3rd 1.46:1
4th 1:1

I will compare this to varios Holden publications I have at home (when i get home tomorrow night)
Feel free to comment....as I think Street Machine have stuffed it up..again

[Dr Terry]

Hi Guys.

The part number that LC-069 quotes gives it away. He quotes '2813498' which doesn't exist, but is easily misread & should be 2813496. This is the cluster gear for the standard ratio (M20) box.

The 2813497 cluster is the M21 type but this could not be misread as 498, could it ?

The ratios that FastEHolden has listed are for Chevy Saginaw boxes & are no relation to the M20/M21/M22 Holden 4 speeds.

Dr Terry.

[Toranavista]

To unreliable Pat. From personal experience.

Any fail safe way to ID them Craig? Would be interested to know, particularly following some drawn out debates on the old site regarding the M22 and/or Rockcrushers. For those that will get excited again, I didn't mention the M22 OK.

The only way to know what you are buying is to remove the inspection cover and check the number on the cluster gear.
M20 is 496
M20 (XUI) is 899
M21 V 8 cluster is 497.
M21 optional L34 and XU1 is 484

Info taken from old forums

[FastEHHolden]

I think that means the doc has confirmed SM have f@cked it up again.

I will post the correct ratios according to Holden when I get home..unless someone beats me.

[LC69GTR]

While we a talking about gear boxes what does the number 5 stamped on it mean and the yellow writing painted on that say's AS what does it mean ?

[Toranavista]

In terms of relative strength the M22 is considered stronger. The strength is attributed to the gears being cut straighter. The downside is it is a noisy box.

[Dangerous]

Yep, forget the grooves. Without taking the inspection plate off, whack it into first, and turn the input shaft while counting the output shaft revolutions. 3.05 turns of the input shaft to 1 turn of the output shaft = it's an m20. 2.54 turns means it's an M21. Can't remember M22 first gear ratio, but it's significantly higher than 3.05:1 - up around the 3.5??. L34 close ratio Aussie 4 speed is 2.32:1, and rare as all Hell (think these ratios were homologated for the XU-1 as well). This info is irrespective of whether it's a 6 or an 8 box.

[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).

2.54/1.65/1.25/1.00 (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.

Hope this adds to the confusion.

Dr Terry.

[Dangerous]

Gotta love it with all that good info in one post. Thanks Doc. One question though - the last gearbox ratios that you quote for the rare option on L34 and LJ XU-1 - shouldn't that have a 2.32:1 first gear ratio, not a 2.54???

Also for clarity, as I understand it, the "XU-1 M20" was standard on XU-1's from the Bathurst 1971 LC production run onwards through all of the LJ XU-1s, with earlier LCs having the Opel box. Is that the case?

[Dr Terry]

Hi Dangerous.

Yes, my mistake, a bit of a slip with the copy & paste. The 1st gear in the last one was 2.32.

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.