Hi Guys,

I have a 69 muncie in my 66. The # on the side of the case is P0M06B which indicates a m21. It has been some time since I drove a m21, but it doesn't feel like a close ratio to me. The Trans I have has some issues that it will have to be removed to address. I have an option of trading my trans and a small amout of money to a guy for a correct trans for my car. The million dollar question is do I go for a m20 or m21. Comments are appreciated on the pros and cons and benifits of each.

Thanks.
Frank

Frank,IMHO and experience an M20 has the better ratios(steeper first gear as well) compared to an M21 and neither one is stronger than the other
M21 is a good tranny if you have 4.88's :D

Hi Chris,

Thanks for the comments. The m20 does have steeper gears in 1st-3rd. 4th is the same in both trans. I guess I am wondering more about durability and the close ratio issue?
Many Thanks!
Frank

The only real difference between the M21 and M22 structurally is the countershaft size. The M20 & 21 have a 7/8", M22 is 1". The only Muncie I ever blew up ( except for syncros) was an M22-go figure.

I have an M21 in my ZZ4 powered '65 Camino and love it. I really like the tight RPM drop in the first two shifts. There is a pretty big drop going into 4th, and if you don't have some torque it may bog a little. My philosophy is that if you haven't got the guy by the time you're grabbin' 4th it's probably too late anyway.

I've run both an M20 and an M21 behind our L78. Give me the M20 any day. For street, especially city driving, the M20 is nicer, not that the M21 is all that bad.

My experiences is that I have to run more gear with a M21 than a M-20.

Not my experiences, but rumors is that for racing a m-21 is better, but for street work and highway traveling a M-20 would be better.

However, I like the whine of a M-22.

ALLLLLLLLLLLLLLLLLLLLLLLLLLL 63-65 Muncies have the 7/8in shaft for the cluster gear (YES, YES, YES, I know about the TINY quantity of 65 M22s, but that is an insignificant factor, compared to the TOTAL of 65 Muncies).

ALLLLLLLLLLLLLLLLLLLLLLLLLLL 66-74 Muncies have the 1in shaft.

ALLLLLLLLLLLLLLLLLLLLLLLLLLL Muncies use the same case/tail housing for their respective years and they ALLLLLLLLLLLLLLLLLLLLLLLLLL use the same internals, EXCEPT for the gears. The difference in the gears is the angle at which the teeth are cut.
For example, a 1968 M20 (wide ratio) and a 1968 M22 use EXACTLY the same case, tail housing, front bearing retainer, midplate, cluster gear shaft, bearings, needle bearings, thrust washers, bolts, synchros, tail shaft, REAR reverse idler gear and sliding reverse gear, side cover and shift forks, rear bushing and seal, etc, etc. ONLY the gears INSIDE the main case are different.
The inherent added strength comes from the loading on the gear teeth of the M22 as a result of the lesser angle of the M22 teeth.

Tom, Arent the M22 gears made for a different material also????

Tom, Arent the M22 gears made for a different material also????

They MAY have a higher nickel content, but I don't know that anyone has absolutely verified that for a fact. If so, then yes, the higher nickel would increase the strength (depending on nickel content). BUUUUUUUUUUUUT, the PRIMARY source of the increased strength of M22 gears (over others) is the tooth angle. When the tooth angle is reduced, the contact area along each tooth, as it meshes with the other tooth, is INCREASED. Thus, spreading the SAME load across a greater area of each tooth.
Does that make sense?
Of course, this also increases the noise factor, which many peope like (I'm in that catagory)! The only thing I like better than a whinning Muncie, is open headers! Especially on a blown fuel motor!

Does that make sense?
Of course, this also increases the noise factor, which many peope like (I'm in that catagory)! The only thing I like better than a whinning Muncie, is open headers! Especially on a blown fuel motor!
Oh I understand totally. I also love the sound of MY M22.

I've pondered the possible difference in reliability between the M20 and the M21, and have a theory that the M21 would be more reliable than an M20 because it deals with approximately 14% less torque multiplication at the maindrive and cluster gears. The rest of the gears (tooth count) are the same between a M20 and M21. The gear ratio at this point in a M20 is 1.19 compared to 1.04 in a M21/M22. The M20 has fewer teeth than the M21 and may be larger, but the additional torque would put more stress on the case at the front. I understand that the Auto Gear Supercase is thicker in this area and supposedly addresses the issue of case stretch. Is there any failure information that could prove or disprove this theory? You guys who work on these transmissions must have some insight into where the failures occur.
I would think a M21 would be more reliable because it does less torque multiplication and usually relies on lower rear gears to compensate.
Bruce

I am very satisfied with my M-20. My car is a driver. I don't want or need big rear end gears. The lower first gear helps a lot with take off. I very rarely do burn outs. The torque issues don't concern me.

Are you sure that the P0M06B is a model number? It looks like it could be a date code. One way to be sure of what you have is to turn the engine by hand and count how many revolutions it takes to turn the output shaft one time. After 30 years or so, a variety of gears sets could have been installed in your Muncie, no matter what the case has stamped on it.

the PRIMARY source of the increased strength of M22 gears (over others) is the tooth angle. When the tooth angle is reduced, the contact area along each tooth, as it meshes with the other tooth, is INCREASED. Thus, spreading the SAME load across a greater area of each tooth.
Does that make sense?



I have also heard another "strength" factor due to the decreased tooth angle has to do with the case spreading (maybe front to back, maybe what Bruce was talking about with the Supercase?) or something, and the straighter-cut gears cause less problems (less thrust back and forth), especially when road racing with lots of "braking" provided by down shifting, which is what the M22 was originally designed for (Road racing Corvettes, right)...

Any truth to this Tom?


IMO, I too prefer the M20 over the M21 for the street, as long as the rear gears are 3.55s or "higher" (3.07, 3.31, 3.42, etc). I ran 4.10s with an M20 for a while, and while it was fun, it got old after a while... Just too low around town (almost a daily driver)... Like driving a semi-truck, shifting before I got across the intersection, etc... I got used to living with the steeper 3-4 RPM drop (now I run a Richmond 5-speed)...

this discussion reminded me how happy i am with my wally built M22W!

Hi Guys,

Thanks for all of the great input and comments. I am running 3.73's in my car. So I am at a catch 22 in some ways. I want a car that is enjoyable to cruise, but has the bottom end if I want to play. So I am leaning towards the m20. DZ, the guy that I talked to I believed said the 7/8 shaft went thru 66 and the 1 in started in 67. In fact he redoes all of the shafts (if you want the heavier shaft) on the 66 and earlier?

Many Thanks to All!
Frank

Tony,
ABSOLUTELY, the SUPER CASE will add strength in areas that were weaker in the original Muncie cases as well as add strength in all directions to reduce stretch. In 40yrs of rebuilding 4spds, I've only seen a very small number of cases that were broken, or damaged beyond repair. The worst repairable damage that I've seen with Muncie cases is an enlarged front hole for the cluster shaft and ears completely broken off. I've only actually had ONE case where the guts came out the bottom front of the tranny. It was a 65 case (for the 7/8 shaft). I had it welded up and had it bored out to accept the 1in cluster shaft, built it with a mixed bag of parts/gears and I think I sold it for $250-300.
But keep in mind, this was all years ago, long before anyone dreamed of offering a Super Case. So, we did what whe had to do to build (or rebuild) 'um!
I still keep a stash of JUNK cases strictly for cutting off an ear to be welded onto a Muncie case that is useable but has an ear that is missing. And it doesn't matter if an ear comes from the top/bottom, left/right side of a donor case. I attach a bearing retainer to the front of a case (for centering in a bell housing), then bolt the case (with remaining ears) to a cast iron bell housing, remove the bearing retainer and use a good C-clamp to clamp down the corner with the missing ear. Bolt the donor ear to the bell housing (with about a 1/8in gap between the case and ear) and have it welded. Once the new ear is welded onto the case, I unbolt the case from the bell housing and have the rest of the ear welded from the front. Once it cools, I VERY CAREFULLY grind and shape the weld to achieve the original contour as close as possible. Lightly blast the welded area, dip the case in a tub of STRONG alum brightener solution and rinse it off with a power washer. Back in business! This is a WHOLE lot cheaper than a Super Case and will allow a matching number tranny to be saved!

Frank,
The FIRST Muncie with the 1in cluster shaft was 66 (the 010 case).

Frank,
If you want the best of both worlds go to an M21Y. You get to choose what gears you want for 1st & 2nd, 3rd is 1 to 1 and 4th is .82 overdrive. I had my M21 rebuilt this way and it runs great. At 75 mph on the highway I'm only turning 2700 rmps. Rear is a 373 and tires are 275/60/15.

I have a M22 with 3.73s. I can start off in 2nd if I want and I sometimes shift 1st to 3rd.

I'd go with the M21. As stated earlier 3.55s to 2.56s are ideal for a M20.

My 65 vette has 3.42s with a M20...works well. JMHO :thumbsup:

Hi Guys,

Good comments and points posted. Now to get a little deeper into getting the right trans. I have decided to go with the M21. The trans. guy wants me to tell him what I want exactly. According to the assy manual the GM # for a m21 is a 3877459. I have been looking around on the web for a GM by the #'s or a Muncie by the #'s with no luck. I just posted a thread in the resto corner about this. So What are the #'s I am looking for as far as the trans goes? He has a bunch of cases and tail shaft's and can build whatever I want.

Thanks Again.
Frank

Your trans is a '70. Should have fine spline input and T400 output yoke.

Why don't you keep it?

Hi Rob,

A couple of reasons. I know I have a issue with 2nd and reverse, not to mention a leaky trans, This guy is going to give me credit for my trans and about 300 for a fresh rebuilt trans that is correct for my car. What would you do?

Frank

If you want a original correct trans for your car I say go for it. You'll need a 3885010 case.

If you want to beat on it I say keep the beefier '70 and up trans you have now and rebuild it.

Either way try to keep original muncie gears. :thumbsup:

Hello Everone and HNY,

Well after a lot of consideration and reading all of the good responses I received in regards to the M20 vs. M21, I decided to go with a M21. Today I finally installed it and got to take it for a ride. WOW! No regrets in going with the 21. Shifts smooth and the close ratio is more enjoyable for me to drive. Many Thanks to all that took the time to respond with good comments.

Sincerely.
Frank
66 SS Big Block