Technical Muncie M20 Question

I am probably going to go with a Muncie M20 wide ratio trans for my A coupe build. It looks like there are 2 versions of this trans, an early one from '63 to '65ish and then one that was built into the 70's. Is there any reason to not use an early version versus the late one other than gearing? Strength wise I think either would be fine in a light model A that mostly sees street cruising. Rest of the drive train will be a hot 283, 9" w/ 3.75 gears and a 31" tire.

Thanks,

--louis

 

The early M20 uses a 7/8" countershaft whereas the later one has a one inch countershaft. The later cases with one inch shafts are obviously stronger, but with your light car you might be ok with the older case if that is what you have. Remember torque shock is what kills these old trannys. I am currently building a '67 M20 for my '56 Chevy build. I purchased a nodular iron midplate from autogear and my countershaft is drilled and tapped for a bolt to go through the midplate into the countershaft for more strength. Hopefully this trans will be able to handle my heavy car and high revving 327.

 

The reason I am asking is that I have the chance to pick up a good used '64 trans but I want to make sure I won't regret it later. I have not bought it yet but the price is right.

Thanks,

--louis

 

I recently acquired a M20 myself, the '63-65 vintage with the 2.56 1st gear and NO grooves on the input spline. The '66 version was a 2.52 1st gear and 2 grooves on the spline. I plan on pounding the crap out of it too, they're a pretty stout gear box.

 

Speedo drive on driver's side thru '70, then pass. side.

 

And you might want to keep in mind that these things are at least 40 years old and what you buy "for a good price" may be parts from several transmissions. This may not be a bad thing, but looking at date codes and part numbers may not be a true reflection of what the tranny is.

 

I just picked up an M20 and I hope it holds up to some rigorous use...

 

I have a 63 model in my vette, no drain plug, I have to siphon out the old stuff.

 

A Muncie trans will take some serious abuse but you can break them. I ran a high revving 301 in my '57 Chevy and in four years of street and strip use I broke two cases, several synchros and some shift forks. As said the biggest change over the years was the increase in size of the countershaft. There are vendors that make steel cases and better gear sets but they are pricey. Good used Muncies around Houston go for $750-$1200.

 

Yeah you can break anything if you try hard enough. The one I am looking at is a numbers matching 1964 that has not been rebuilt but no visible damage for $500ish. Found a B&W T-10 for a little less but I think those were all close ratio like the M21s.

--louis

 

is the front shaft on them supposed to wiggle a bit? or no?
-Pat

 

the input shaft is held by a ball bearing, so it's ok if it wiggles a little.

 

I have an M-20 that I intend to put behind a Buick 401 in a Model A. Am I just courting disaster? I really wanted 3 pedals instead of running the 400 auto.

 

The T 10 have a lower first Gear

 

I have an early M20 behind a 455 Olds in a fenderless'30 A Coupe. Holds up just fine, and I am not kind to it. Damn I like bangin' gears!
I did drill and tap a drain plug hole - dunno why any engineer designing a tranny, manual or automatic, would not include a drain lug.

 

If your car is light and you aren't beating it to death, an M20 will hold up fine IMO. Pounded M20s & M21s in heavier cars like A and F bodies as a youngster and can tell you they will take a lot.
Yep, the lack of a drain plug on the early ones must have saved GM about 99 cents per car.

 

Just be aware of false economics. For your intended use in your application, a used M20 will probably work just fine. I put a swap-meet M20 (vintage 1970, $650) behind the SBC in my 37 Ford, and scattered it on my first pass at Meltdown Drags 2010. Pictures are in my album. It is sitting in a box of broken parts in the basement.

I replaced it with a new wide-ratio M22 from Mediatronics Corp.; that has held up fine. I should have started with that to begin with.

 

I have three late 60's wide ratio Muncies under the work bench.All the gears have worn synchro engagement teeth and sliders. One twisted mainshaft.and two of the cases have worn counter shaft bores....I built a fourth using the best parts that works well enough for street use. These are all swap meet items bought over the years and I believe typical of most Muncies.

 

In the '60s I ran M-20 and M-21 4-speeds in my '55 Chevy Gasser. We didn't have steel mid plates to add strength to the case. I only broke one of the ears off the trans and learned that there were two important issues with those gearboxes. The register hole in the bell housing must be checked for runout and a transmission crossmember is required to help eliminate the shock of dropping the clutch at 8000 rpm. If the brass blocker rings are in good shape and all other parts are not worn the old boxes are more that capable of reliable street driven hot rods. I currently have a M-20 in my 3W and I do still power shift it, no problem!!!

 

Piece of cake to hoist the car, drill out the case and pipe tap for 3/4" pipe plug. Aluminum shavings drain right out...with the 80W gear oil!

 

Wrong

T-10 2.20 first or 2.54 first

 

Pat,

If the input shaft did not wiggle then there would be very few cars that the transmission would work in without serious problems due to bell housing pilot bore not being concentric with the crankshaft pilot bearing/bushing. You can always tell when there is a significant concentricity error because shifting into high gear and keeping it in high gear becomes a problem. If you want to do it right then dial indicate the bell housing pilot bore using the crankshaft as the rotary axis. They make offset dowels for correcting the error.

Ron

 

If my memory serves me correctly the first Muncie was rated at 400 ft/lbs of torque. This was 1964.

 

Thanks all for the info. I wasn't alive when these were new and I understand that 4 speeds were in Buick GS's, but I haven't been able to ferret out the specifics on the how and whys.