M 20 6Cyl Verses V8

[dags061]

I know that this has probably been discussed on this forum but i am getting conflicting info. Is it the input shaft or the input nose the only difference between 6 and 8 boxes.I thought that you changed the input nose and then its all hunky dory.Could someone please clear this up.I know bellhousing ,clutch ,flywheel are different. Cheers Darryl

[Glyn]

The imput shaft is longer in the V8 transmision than the 6 cyl types. Fact. They are however interchangeable. I think from memory the thrust bearing spigot (what you call nose) is longer also. They are not difficult to change over if you can get the parts. I would count the gear teeth to ensure the ratio is correct.

[Hangman]

As above, boxes are the same except for the input shaft, but it's all interchangeable.

[Smitty]

the only diff is the clutch (or input) shaft
and the clutch shaft bearing retainer

both are longer in the V8 version

[dags061]

Thanks everyone. I currently run an M20 behind a 202 and was thinking of putting it behind a 253.Spoke to a guy I know at the wreckers and he cleared it up.Would probably better off getting an M21 anyway.

[Hangman]

Just so you know, if you don't already, the only difference is the ratios between the m20 and m21. The m21 isn't stronger or anything.

[dags061]

I knew the ratios were different and was under the illusion that M21s were stronger as they were put behind 308s and XU1s.Thanks for the heads up.

[Glyn]

Easy mistake to make.
The only difference between the M20 and M21 is the first gear ratio. There is a way to tell them apart.
Have a look at the imput shaft spline. At the start of the spline you will see a groove machined in.
If it has 1 groove, you have a M20. Two grooves then a M21. If it has 3 grooves then you are a very lucky boy as that is a LJ XU1 tranny and is Gold rocking horse sh!t. Early HQ 1ton utes had a differnt 1st gear ratio again but I cant remember the identification.

Ausie 4 speeds are a good solid trans for 6's, as long as you dont miss too many gears when driving. They should never have been put behind the V8's as they can't handle the torque if driven hard. I used to overhaul them for a bit of a hobby and have a lot of torn up boxes in my shed.

A couple of tips if looking to buy second hand.

Ther are two main volnerabilities

1 The dog teeth on the gears are quite brittle. Take the selector cover off and have a close look especialy second gear. If the dog teeth look a bit chipped or rounded off, it will jump out of gear. Real pain in the arse.

2 The cluster gear internal bearing is prone to wear. This makes it noisy in 1st less noisy in 2nd and so on. You can have the cluster resleeved and is a good repair but if it was driven too long when worn out it will still be a bit whiney when overhauled.
The cluster bearings are not that easy to check unless apart. Metal paste in the bottm of the gearbox is a dead giveaway. Grab the cluster at the imput shaft end and give it a good solid shake. It should have next to none radial clearance. expect a little axial clearance.(end float)

Sometimes the gears seize on the main shaft or the main shaft may seize in the imput shaft. this is less common. A nice tight Aussie 4 speed is a bit of a pig when cold and people force gear changes, This is where a lot of wear takes place. I found a tube of moly-bond in the oil helped a lot with this. There are Quite a few good oil additives around these days.

Hope this helps

[SL/ENUT]

Sick to death of people saying the only difference is 1st gear ratio, its not.

Here is the M20 gear ratios( V8 M20 version same)
1st: 3.05:1
2nd: 2.19:1
3rd: 1.15:1
4th: 1.00:1
Reverse: 3.05:1

This is the M21 gear ratio
1st: 2.54:1
2nd: 1.83:1
3rd: 1.38:1
4th: 1.00:1
Reverse: 2.54:1

Source, GMH VB Commodore service manual.

Also my M21 behind my mild 308, has done the job well, still hasn't been rebuilt. Only a seal replaced.

[Smitty]

hmmm...some myths and legends here

the XU1 4 speed was always ..M20 on the build sheets
and came standard with different ratios to the V8 M21 box
but dont forget there were 2 others boxes homologated for use
with the XU1..the normal 6 cyl M20 used in the GTR and the
box with the same ratios as used on the L34...with a 2.32 gear set

XU1 boxes may have 1 groove 2 grooves or...NO grooves in my experience
the real check as the number of grooves can be misleading is the cluster gear part number

M20 normal ends with 496 M20 (XUI) is 899
M21 V8 cluster is 497. M21 optional L34 and XU1 is 484

as mentioned ..the cluster gear internal bearing surface is prone to wear..but
this is not the real issue. That is one example of what happens when the case
hardening gets worn through. The Cluster gear is one example...it also happens
to other case hardened parts. Part of the reason the cluster comes up first
with this fault...is that the internal oiling for the needle rollers in the cluster
is crap... One small oiling hole means little gets in...and then some idiots
put in ATF to fix up (they reckon) cold shift issues and it wont protect
the cluster (or any internals from wear) The suggestion of molybond
is an oldtimers fix for cold shift issues...

but a properly built / rebuilt aussie box has to have minimal clearances(or end floats)
and that means GOOD bearings (no chinese crap) and time...time to fix endfloat
and clearances. (Motor cycle valve shims are nice behind the bronze bushes)
and properly built, it will happily run a 140w gear oil and have no shift issues.
I build and run this sort of box in my race car and (with an annual tear down and check)
I have had 4 seasons out of the latest

[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.