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Old 09-08-2005, 01:31 PM
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Which Muncie Do I Choose?

Hi Guys,

I am looking to rebuild one of my Muncie transmissions for my 69 camaro. I have an M20 that pops out of 4th gear (needs new syncros) and I have an M21 that grinds in 4th gear (needs new syncros). They are both early Muncies. My questions is which one is the better candidate for rebuilding? I know the rebuild is the same but is any stronger that the other? If not why did they make both? Also does anyone know of these Super Muncie cases with plugs for the counter-shaft, ect. Just curious. Maybe there is a better 4-speed instead of these two? Any thoughts are very appreciated. BTW, The Camaro has a built 327.

Thanks Jim

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Old 09-08-2005, 01:51 PM
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muncie

The m-20 and m-21 muncie are about the same as far as strength goes. the m-20 might be a bit stronger due to the gear ratio design at the input to custer contact but thats even questionable... Both transmissions use the exact same 1st ,2nd and 3rd gears, they are interchangable... Aslo they use the same case, main shaft, bearings etc.... the only difference is the input and cluster gear teeth count......

the reason for the 2 different transmissions was the ratio's
the m-20 is 2.52,1.88,1.46,1.0
the m-21 is 2.20,1.64,1.28,1.0

So you can see the m20 has a lower 1st gear but a bigger spread between the gears. the m20 would be better suited in a lower hp car with a 3.55 gear or so to help in the lower end... the m21 works better in the higher hp cars with like 3.73 4.11 and so on.... Alot of it comes down t0 preference....

Either one would work for your combo... The super cases are very nice but if your cases are good with no broken ears and the counter pin is tight i would just run the stock case....

Keith
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Old 09-08-2005, 02:25 PM
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Thanks for that great information. The M21 was the one that I was going to use but the counter-shaft on the M21 leaks. I have never had issues because I use a little RTV and gasket material and smooch it against the bell-housing. But now that you mentioned that I will have to give it some thought. Suggestions? Thanks again

Jim
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Old 09-08-2005, 03:52 PM
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Not the same...

The gears on these are not interchangeable except as a SET from one case to the other.

The M21 is somewhat stronger only due to the gear ratio being lower (2.20 versus 2.52 or 2.56). This is because gears multiply torque, so if you apply 300 lbs/ft to an M21 the internals will be stressed 2.2 times that, versus 2.52 for the M20. Given that the gears are the same materials and roughly the same size, the lower stressed box would be a tad stronger.

The reason for the two ratios was to accomodate different rear axle ratios. 3.55 and up are OK for the M21, while lower (numerically) are not so good. Example, if you had a 3.31 rear with an M21 you'd bog off the line, but be able to do 50 in first.

The so called "wide" ratio M20 works fine with 3.31, 3.36 and the like. Still not good with a 3.08. But with a 3.73 or 4.11, the "wide" ratio M20 wouldn't space out as nicely as the "close" ratio M21.

Later on, Sags and ST10's came along with better street ratios. Example: a 3.11 first gear Sag is just right for a smallblock with a 3.08. 2.64,2.88, and 3.42 1st gear ST10's came along. Note the 3.42 is EASY to break, as you can imagine by multiplying torque as mentioned above.

Anyway, your 327 will survive just fine with either M20 or M21. I'd pick based on what rear axle ratio you have. So, what's yours??

Tom
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Old 09-08-2005, 06:34 PM
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Well, you want the toughest muncie, sell the 2 others you have, find a rebuildable m22 rock crusher, you will thank yourself for the extra strength in the long run.
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Old 09-09-2005, 06:09 AM
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muncie

Tom, please explane why you can't swap the 1st,2nd and 3rd gears??? I have been doing it for the last 20 years and never had a problem.... never had a noisy trans or any parts breakage....They are even the same Gm part number......

The torque multiplication does play a small roll in the strength but the root diameter of the gear is a bigger factor in the strength of the gear.... the m21 has a 26 tooth count input the m20 has a 21 tooth count input... So in order to squeeze the extra 5 teeth in there the root or pitch diameter of the gear needs to be smaller, that is where the weekness comes in....

Keith
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Old 09-09-2005, 06:39 AM
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Thanks again guys this is some great information. To answer Fleet49's question the motor is a built 327 dynoed at 390 hp with 3:73 gears. Also last night I pulled the number off of the spare trans and I have listed them below. Can anyone verify that it is an M20 that is what I was told. Thanks again I love this site what a resource.

Also, The trans in the car I am not sure of the numbers but I know it is an earlier Muncie because the speedo gear is on the drivers side of the trans.

Based on all of the information which one do I use.

388 on side of case
3885010 on case
P0519
3857584 GM on the tail
6B176243 on top of case
3851326 on front of case
It has 2 input grooves
and a 1" shaft the trans in the car has the smaller shaft

NOTE: Now that I have been doing a little more research I think the M20 out of the car would be the better fit. with the larger 1" shaft and being a new year etc, what do you guys think?

If thats the case where can I get a good rebuild kit and what should be included in the kit. Preferably someone with good customer service.
Jim

Last edited by JimKramer; 09-09-2005 at 06:57 AM.
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Old 09-09-2005, 07:00 AM
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388 on side of case-------------not sure what this is

3885010 on case----------------------------66/67 case

P0519--------------------------------------starting in 67 there was a model year id number in this code, because your code is missing the 7 in there then that makes it a 66 transmission

3857584 GM on the tail----------------------correct tail for a 66/67 transmission

6B176243 on top of case----------the 6 is the model year the B is the plant it was built at, and the other 6 digits are from the orginal cars vin

3851326 on front of case-----------------correct bearing retainer for a 66/67 transmission

It has 2 input grooves------------if it's never been apart then the 2 rings denote a m-20

and a 1" shaft the trans in the car has the smaller shaft------------- if the other transmission has the 7/8" counter shaft then use this transmission the 7/8" pin transmissions had some issues. this transmission is a stronger piece...

Jim[/QUOTE]
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Old 09-09-2005, 07:09 AM
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rebuild kit

I sell the kits, PM me if your interested,, or do a web search for them....

Make sure it's a complete kit.. there are some kits out there that are $25.00 cheeper but are missing some key parts...

Keith
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Old 09-09-2005, 09:21 AM
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Quote:
Originally Posted by k-star
Make sure it's a complete kit.. there are some kits out there that are $25.00 cheeper but are missing some key parts...
Keith

Amen brother, I can vouch for that , or they have Chinese bearings.

Vince
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