Look in the post at the top on the trans fourm. It tells how to pick out what is what but the main thing to look for in a M-22 is the gear angle. Take off the side cover and look at the angle of the gears. M-20, m-21 will have 45 degrees gears and the m-22 is 22.5 degrees. They used this to reduce friction and in turn gave it a nice greary noise thus the rock crusher.
The famous "rockcrusher" M22 4 speed had the same gear ratios as the M21 but with a heavy duty gearset. Although the gears are not spur gears ( completely straight ) they still produced a fair amount of gear noise thus the "Rockcrusher" name.
The main reason for decreasing the gear angle on the M22 was twofold. First it allowed the gearset to run cooler. Second it reduced the end loading on the mid plate. The end loading on the mid plate was the primary reason for the angle change. Contrary to the popular tale, the gears are not "straight cut". The only way to tell an M22 is by looking at the gear set. You cannot go by the old method of "it has a drain plug so it's an M22". Also the input shaft cannot be used as a guide because there are various firms now manufacturing replacement input shafts that have no identifying rings. A 100% original M22 is a rare beast, as most have had various gears replaced, and some M22's have been created by installing entire M22 gearsets in an M20 or M21 case.
The only way to be 100% sure is to pull the side cover and check the gear angle like the others have stated. Depending on the year of the transmission there are some things to look for, but as vince stated if the parts were swapped this will not hold true.
The input should have no rings around it.
If it's a 1970 or newer the code on the side of the trans will end with a C
Also all the m22 i have seen that were un molested have had 2 plugs on the passenger side. one is a fill and the other a drain.
A true unmolested m22 is almost none existent today. Can you post a picture of it for us to help you out???
I don't own it. I am going to be getting one soon. I was just wondering if I could tell. I don't want to get the transmission to come home and find out that its actually a M-20 or 21. If i took the side case off. i will need to get a new gasket right?
The gear set can be a little difficult to identify by someone who is not familiar with it, especially if you do not have an M20 or M21 gearset to compare it to. You will have to rely on the pictures that have been provided here.