M 20 6Cyl Verses V8

[Hangman]

While we're getting technical, I pulled this off another forum. Good info here.

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.

And a bit more info in a reply to the above.

The thing not mentioned 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.


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).

 

[Glyn]

Enjoyed reading the above posts. Its been 30 years since I seriously played around with these things so I was generalising a bit in my previous post.
Molybond a old timer fix, Hell yeah.

One thing I realised early on is that the part no system could not be trusted when obtaining replacement parts. In those days spare parts at the dealers would stock trans parts and I would cruise around with a set of vernier calipers and buy anything with phisically the same dimentions. There may well have been differences in metalergy etc but I don't recall any major problem. I would expect the part no system became complicated due to ongoing development over the years.

Another thing that can't be trusted is the technical manuals as far as what trans was fitted it what model. I know from people who worked on the production line in the 70's that the all important thing at the time was keeping it running and if the correct part wasn't available and something else was at hand that would fit in it went so the producction line didn't stop. This is how my wifes V8 VB ended up with a 4cyl radiator from the factory.

The only reason I mentiond the 3 rings on the GTR XU1 trans is that a freind of mine has one that he bought brand new in '73 and I noticed it when we did a clutch service. It is obvious that the imput shaft rings can't be trusted for identification as these gear-boxes have been around for 40 years now and blokes like me have been building up serviceable units from boxes of mixed up bits. One thing they don't lack is interchangeablity.

Apologise for the mistake on the ratios. seniors moment.

The guys that have these boxes behind V8's with no issues, Just means your good drivers. I know people who could kill it in a week.

 

[SL/ENUT]

My car does get the odd flogging(clean out the "carbon") lol.

 

[dags061]

Well now I am well informed. I am no hoon and in that case an M20 would probably last a long time.Thanks all.

 

[Hangman]

You can break any box if you try. I'm yet to break one, but I don't dump the clutch everywhere I go either.

 

[MickydaFish]

Enjoyed reading the above posts. Its been 30 years since I seriously played around with these things so I was generalising a bit in my previous post.
Molybond a old timer fix, Hell yeah.

One thing I realised early on is that the part no system could not be trusted when obtaining replacement parts. In those days spare parts at the dealers would stock trans parts and I would cruise around with a set of vernier calipers and buy anything with phisically the same dimentions. There may well have been differences in metalergy etc but I don't recall any major problem. I would expect the part no system became complicated due to ongoing development over the years.

Another thing that can't be trusted is the technical manuals as far as what trans was fitted it what model. I know from people who worked on the production line in the 70's that the all important thing at the time was keeping it running and if the correct part wasn't available and something else was at hand that would fit in it went so the producction line didn't stop. This is how my wifes V8 VB ended up with a 4cyl radiator from the factory.

The only reason I mentiond the 3 rings on the GTR XU1 trans is that a freind of mine has one that he bought brand new in '73 and I noticed it when we did a clutch service. It is obvious that the imput shaft rings can't be trusted for identification as these gear-boxes have been around for 40 years now and blokes like me have been building up serviceable units from boxes of mixed up bits. One thing they don't lack is interchangeablity.

Apologise for the mistake on the ratios. seniors moment.

The guys that have these boxes behind V8's with no issues, Just means your good drivers. I know people who could kill it in a week.

Hi Glyn

Curious about this post. I am wondering about this 3rd ring on XU1 box, is it next to the other two or further back just near where the input shaft cover ends?

 

[vc commodore]

Hi Glyn

Curious about this post. I am wondering about this 3rd ring on XU1 box, is it next to the other two or further back just near where the input shaft cover ends?

He hasn't been on here since early 2014, so doubt he'll answer

 

[MickydaFish]

ok, couldn't forward me his email I guess?

 

[vc commodore]

ok, couldn't forward me his email I guess?

Don't have it to do that for you.

If you click on his name, you will see when he was last active....That how I know when he was last active