How to tell the difference between M20-21-22

[Michael J.]

After talking to all the former owners of my '67, I am now not sure what tranny I have in it. It is supposed to be an M21, according to the tank sticker, but several previous owners tell me they blew trannies and replaced with M22. Is there anyway to tell which I have in it now without taking it apart and counting teeth, etc.? Thanks.

[Wayne M.]

Re: How to tell the difference between M20-21-22

Step # 1 would be to figure out wide ratio (M20) vs. close (M21 & 22). Note RPM's in 3rd gear and 4th at same speed in each gear. If RPM ratio 3-to-4 is 1.48 (or close) then you have an M20. If only a small difference when shifting 3-to-4 (1.28 RPM ratio, or close) then you have an M21 or 22 [same tooth count for these two close ratio trannys]. For M21 versus M22 -- drive around in 1st gear (say 15 to 30 mph ) under light throttle. If the neighbors come out and throw beer bottles at you, then you have a rock crusher.

If you can get the car up on a hoist, there's a bunch of casting #s and stampings that could tell you what you have, depending on when it was assembled, and assuming that the internals have not been replaced. At the same time, as a bonus, you could check what's stamped on the bottom case of your differential.

[Michael J.]

Re: How to tell the difference between M20-21-22

OK, I'll check the ratios, but my neighbors love that L71 sound through the side exhaust, but guess you mean the tranny should be whining and making more noise than the engine. Mine doesn't, and I many times wind it out to 50 or so in 1st, I have the 3.55 rear. But I will put it on the lift and look at the numbers closely. I haven't done that since I got my lifts installed anyway.

[John H.]

Re: How to tell the difference between M20-21-22

Michael -

There are no numbers on a '67 Muncie that will tell you anything about which gearset it had originally; aside from the extremely obvious whine in 1st, 2nd, and 3rd, the only way to tell for sure is to pull the side cover off and look at the helix angle of the gear teeth - M20/21 gears are about 40*, and M22 gears are about 20* (see photos below). The Muncie "P"-code didn't have the ratio suffix until mid-October, 1968.


M20Gears800.jpgM22Gears1024.jpg

[Larry M.]

Re: How to tell the difference between M20-21-22

If I recall correctly, the M-22 had both a fill and a drain plug in the main case. The M-20/M-21 only had a fill connection. (You used a syphon gun to remove the fluid )

Larry

[John H.]

Re: How to tell the difference between M20-21-22

If I recall correctly, the M-22 had both a fill and a drain plug in the main case. The M-20/M-21 only had a fill connection. (You used a syphon gun to remove the fluid )

Larry

Larry -

That's correct (through 1969); however, many Muncie rebuilders drilled/tapped the existing boss and added a drain plug as a convenience, so you can't always tell by the presence of a drain plug.

[Wayne M.]

Re: How to tell the difference between M20-21-22

Here's and example of the assembly and suffix code that John mentions, located on the passenger side, on the rear stamp pad at the junction of the maincase and the tail housing. This one shows P1D19C, which translates Muncie IN, 1971 model year, April 19th, C = M22 [A= M20; B = M21].

The 5-digit number, 43265, above and to the right is the numbering system for a GM over-the-counter Muncie transmission. Believe it started at 35000 each year. To the left of that # (not shown) is stamped "CT1" which is Chevrolet Transmission, 1971.

[Michael J.]

Re: How to tell the difference between M20-21-22

Thanks for all the tips, I'm pretty sure it is not an M22 now, as it is very quiet and smooth through the gears, but I will check the RPM ratios and get her on the lift to look at the numbers. Thanks again, guess they may have put an M21 back in when it was restored and stamped the tranny with the VIN, but I have not checked the other codes.

[Timothy B.]

Re: How to tell the difference between M20-21-22

Michael,

A good way to check the ratio is drive to 2200rpm in first gear then shift to fourth gear and the rpm should be 1000. If it's not then drive to 2500rpm then shift to fourth and seeif it's 1000.

2200rpm= m-21, 2500rpm=m-20

[Jim L.]

Re: How to tell the difference between M20-21-22

I'm pretty sure it is not an M22 now, as it is very quiet and smooth through the gears,

If it is an M22, you would know this immediately from the growling sound the gears make in 1st gear. The noise is impossible to ignore.

The car in my avatar has an M22. You can hear the 1st gear noise in spite of the sound of the (nearly open) side pipes, if that tells you anything.

Timothy Barbieri's technique for determining the difference between a wide ratio and a close ratio transmission is absolutely definitive. It's what I do when I have any doubts.

Jim

[Tom D.]

Re: How to tell the difference between M20-21-22

I have a metal strip (ID tag) that was on a 69-70 Muncie. The number has a suffix = "WD". Suppose that means anything... in the context of this thread?

[Tom P.]

Re: How to tell the difference between M20-21-22

Michael -

There are no numbers on a '67 Muncie that will tell you anything about which gearset it had originally; aside from the extremely obvious whine in 1st, 2nd, and 3rd, the only way to tell for sure is to pull the side cover off and look at the helix angle of the gear teeth - M20/21 gears are about 40*, and M22 gears are about 20* (see photos below). The Muncie "P"-code didn't have the ratio suffix until mid-October, 1968.


[ATTACH=CONFIG]37724[/ATTACH][ATTACH=CONFIG]37725[/ATTACH]

John is correct.

[Tom P.]

Re: How to tell the difference between M20-21-22

Larry -

That's correct (through 1969); however, many Muncie rebuilders drilled/tapped the existing boss and added a drain plug as a convenience, so you can't always tell by the presence of a drain plug.

Again, John is correct. Beginning in 70, ALL Muncies got a drain plug.
Every Muncie that I rebuild, I install a drain plug. In 45+yrs of rebuilding Muncies, I have had ONE customer request that I not install a drain plug.

[Terry M.]

Re: How to tell the difference between M20-21-22

I have a metal strip (ID tag) that was on a 69-70 Muncie. The number has a suffix = "WD". Suppose that means anything... in the context of this thread?

There should also be an ink stamped WD on the transmission (usually on the top so it is not visible as installed). That number is the transmission part number and I suspect if we could decode it we would know not only the type of transmission but the speedometer gear that was installed. The WD is the broadcast code used by the assembly plant workers to pull the proper transmission for your Coirvette. Again, if we knew those codes we could tell the type transmission and speedometer gear installed.

[Wayne M.]

Re: How to tell the difference between M20-21-22

Michael -

There are no numbers on a '67 Muncie that will tell you anything about which gearset it had originally; aside from the extremely obvious whine in 1st, 2nd, and 3rd, the only way to tell for sure is to pull the side cover off and look at the helix angle of the gear teeth - M20/21 gears are about 40*, and M22 gears are about 20* (see photos below). The Muncie "P"-code didn't have the ratio suffix until mid-October, 1968.


[ATTACH=CONFIG]37724[/ATTACH][ATTACH=CONFIG]37725[/ATTACH]

Here's two third gears, side-by side, to show the angle difference that John mentions. Because of the shallower angle on the M22, it permits a wider tooth face, which I believe contributes to the strength of the gear. Also, I've heard that the M22 runs cooler, due to the reduced sliding friction of engaged teeth of whatever gear is under power.