what is the difference between the m20 and m21? i know the difference of a m22 but what is the difference of an m20 to an m21? which is a stronger tranny?

The difference is the gear ratios. I don't have them all memorized, but the M20 is a "wide ratio" box with a 2.52:1 low, the 21 is "close ratio" with a 2.2:1 low.

which is a stronger tranny?

No difference from what I've read. It really came down between the M20/M21 and the M22

An M-21 is some stronger b/c of the lower first gear!

pdq67

M20 ratio is 2:52, 1:88, 1:46, 1:00. An M21 is 2:20, 1:64, 1.28, 1:00. I run a M21 with a 4:10 rear and I also have a M20 with a 3:73. This combo is almost identical in 1st and 2nd but the M20 with the 3:73 is better on the highway.

right now i have a saginaw 4 speed with a hurst competition plus shifter & linkage. should i ditch the saginaw eventhough it is fully re built for a muncie m20 or m21 thta isnt rebuilt but in good shape? also which one should i get the m20 with the 2:52 first or the m21 2:20 i run a 3.08 rear and i have a hard enough time taking off with a 2.54 first.

with a 3.08 rear, you want an m20 for sure.
no reason to ditch the sag until you bust it tho. Probably better things to spend coin on than replacing something that still works.

I've never had a Saginaw but I guess it all depends on how you drive your car. It's probably fine for a driver but from what I've read I don't think there is any comparison as far as strength between that and a Muncie. If your keeping the 3:08 I would go with the M20. A M21 would probably boog down the car in 1st. You may want to consider a slightly different gear in the rear. Maybe a 3:55, or 3:31.

was the m20 for small block cars, and m21/m22 for big block cars, due to the diferent gear ratios and strength?

it prob was, like the 3.50 1st saginaws were for the L-6 cars. my friend has a 2.20 first in his 350 chevelle with 2.73's! wow i wonder how his take off is? he said he can go 106 mph @3,000 r.p.m thats just too low rpm for me.

You will find all of them in almost any car. Most of the time the M22's were in the big blocks as they are pretty much bullet proof and could handle the high hp and tq.

Neither of the 3 muncies were BB/SB specific. Could be had in either, with the 22 being only in the H/P applications.

The 20 has lower ratios (and drops alot 3-4), the 21 ratios are closer to each other. I dont think there is any significant strength difference between the two.

I have a 2.88 low ST-10 in the old Nova wagon and a 9in with 3.00s. If you ever see any traffic I wouldn't go with any less first gear ratio than I have. Even with a very mild 350 it is right on the edge of being a pain if you get stuck in traffic going up a hill. Most Saginaws you find have a 3.11 low gear that works well with 3.08s. There can be a problem with the Saginaw they simply won't take any abuse. Another choice is a 3.42 low ST-10 they are the most common and can be had cheaper than a Muncie but you will need to change the clutch and shorten the driveshaft.

Did you say it was a 2.54 Sag??

If so, it's the strong one.

And I think you could option any of the three Muncies behind ANY motor back then! 6-banger w/ an M-22 probably isn't possible, but we never know???

pdq67

My cross ram 69Z has a custom gears in the muncie trans. 296 first 221 second 188 third with 373 gears. Its great for the street. Its got a wide space between 3-4 but it doesn't seem to matter . My Big Block car has a M20 and 355s and thats A nice setup.

was the m20 for small block cars, and m21/m22 for big block cars, due to the diferent gear ratios and strength?

M20 was the "standard" 4 speed (got the car moving easier and saved on clutches). M21 was more for racing (road and drag) for keeping in the higher rpm power band. The M22 had stronger gears because of the smaller helix angle on the gears.

How do you tell the difference between them visually? I have one out of my 67 for sale but dont know which Muncie it is. Do I have to count gears or splines or are there #'s on the case?,,,,,, Then I have to figure out what its worth......

How do you tell the difference between them visually? I have one out of my 67 for sale but dont know which Muncie it is. Do I have to count gears or splines or are there #'s on the case?,,,,,, Then I have to figure out what its worth......
get the casting numbers off of the case and post them here. also if it is an m22 you can visually tell beause of the shallow gear angles over the straighter m20/m21.

Did you say it was a 2.54 Sag??

If so, it's the strong one.

And I think you could option any of the three Muncies behind ANY motor back then! 6-banger w/ an M-22 probably isn't possible, but we never know???

pdq67
yes it is the 2.54 first saginaw they say they made 2 and i have no idea which it is, i have posted all the casting numbers with no luck. i traded a mint borgwarner for the saginaw and a hood :clonk: but i was in desperate need for a hood so i had to do what was right and i didnt really need the tranny. also i was wondering which of the 2 muncies (m20/m21) would have the least rpm shift drop my math says the m20?? also how much would be a fair price to buy a used m20 with linkage no shifter from a friend?? would a 2.54 saginaw 4 speed do good if i raced it? the shift drop is close to the m20 but drops more between 2nd and 3rd i think?

The reason an M-21 is stronger than an M-20 is the input to cluster ratio...
The M-21 has 26 teeth on the input gear and 27 teeth on the cluster gear...
The M-20 has 21 teeth on the input gear and 25 teeth on the cluster gear...
The closer the tooth counts are the stronger they are,,, thats why a ST-10 3.42Z is so weak with 17 teeth on the input and 31 teeth on the cluster...
Hope this helps... Crash

thats some good info, so i see now that super T-10 you listed is weak i was prob better off selling the super T-10 eventhough i was told its strong unless it was a different type. anyone know how many teeth a saginaw 2.54 has??

M20s, M21s and M22s are all VERY strong transmissions . . . Sags are not even close.

I ran an M20 (69 version not the 71-74 HD version) with a BBC putting out 425hp with an additional 250hp shot of nitrous and never broke the trans (4.89 chrystler 8 3/4 posi rear).
Of course if you get complete hookup at the drags this could change how long things last!!!
All M22s are not created equal. The early versions had course splined clutch imput shafts where as the later versions had the fine spline input shafts. The 22s also had a turbo 400 size rear yoke (larger shaft size) in the rear. And they all had the 1" diameter counter gear shaft where as the 20s and 21s had a 3/4" counter shaft diameter. This is one reason the 22s are stronger.

I believe another reason the 22s are stronger is because they used heat treated metal for the gears inside them with the different angles on the teeth when compared to the 20s and 21s.

In the 1966 model year all Muncie 4-speeds started using 1" cluster shafts(M-20's,M-21's,and M-22's)
1965 thru 1970 M-22's used 27 spline output mainshafts and 10 spline inputs...
1970 thru 1974 M-22's used 32 spline output mainshafts and 26 spline inputs...
In 1970 you get M-22's either way...
In 1963 thru 1965 Muncie's used 7/8" cluster shafts...
Hope this helps... Crash

Crash
good info.

Also, ALL 71-74 M20, 21,22s have the 32 spline output mainshafts and 26 spline inputs.