JLMvette73

How, other than taking apart the tranny, can the ratio on a Muncie transmission built prior to October, 1968 ( when a suffix code which identified the ratio was added to the assembly date code) be determined?

The Muncie I have in my '68 has the following castings:
Maincase:3885010
Extention Housing:3857584
Sidecover:3884685
assembly date Code:P7P28, which decodes to 4-speed Muncie, 1967 model year, September 28th.
There is also a tag on the transmisson stamped with the number 870357. As of yet, I have no idea what the number on the tag refers to?

Unless I've overlooked it, I've found no reference in my "Corvette By the Numbers" or the assembly manual referencing this tag nor have found anything to help identify which Muncie I have, as all 3, M20, M21 & M22, used the same maincase, extension housing and side cover casting number in 1967.

Any help would be appreciated.
Thanks,
Jerry

crazydalys

I'm not sure, but I think there might be markings on the input shaft.

ImBatman

checkout www.5speeds.com they should be able to help you.

PRNDL

you can easily determine the ratios by driving it. If your car is not driveable, you can stop right here! If it is just determining if trans is M20 or M21 you drive in 4th gear and downshift to 3rd, keeping your speed constant, noting the change in RPM. But you can actually figure out all of the respective gear ratios by checking the rpm changes between gears.

JLMvette73

5speeds.com references the # of teeth and rings on the imput & output shafts. With the tranny being in the car at this time, I may just have to resort to removing the side cover and see if I can count the teeth on the imput gear and cluster gear. One interesting thing is how short the throw is between gears, for a Muncie shifter. Haven't even come close to hammering my knuckles on the dash.

ImBatman

I have my Muncie apart right now doing a rebuild. It is a 64-65 model M20 gears are 27,30, and 36. Did you look up the whether it is a 20 or 21? I have a parts schematic at home for all 3 and if you have a 20 I can reference mine if you have any questions so you want have to take the cover off. Let me know.

JLMvette73

BlackRat, how do I "look up" whether it's an M20 or M21? The M20 & M21 (and the M22, for that matter) built in 1967 all use the same maincase, tail housing and side cover casting numbers. What else am I looking for on the tranny to help identify it? Maybe the #870357 on the mysterious tag on the tranny that I can't seem to find any reference to anywhere......

435Randy

The number you are refering to is the part number. This number will tell you what the engine and trans is, such as M21, 300 horse. In the "67" AIM it gives the description of the numbers, you might want to look again. If I was at home and had my books I could help you more.

Randy

stingr69

The metal tag is the part number for the assembly. You could have different gears inside the case though if it has been apart before or if the tag is not original.

Put it in first gear, mark the output shaft and input shaft, then rotate the input shaft while counting the rotations untill the output shaft has rotated 1 full revolution.

If the input shaft has rotated 2-1/2 rotations for 1 rotation of the output shaft, you have a wide ratio M20.

If it is less than 2-1/4 revolutions on the input for one revolution of the output, you have a M21 close ratio (or M22).

-Mark.

ImBatman

I will get you the exact location of mine when I get home. although the cases are the same there still should be an identifying # on your case to reference.

JLMvette73

BlackRat, Randy and Mark, thanks for the help!!!

mike229

Never mind - I just saw you referenced the suffix code but that didnt help you w the pre 68....

PRNDL

so is the car with the transmission in driveable?

Here is the procedure in detail. This was explained to me by a forum member when I had the same question, and easily determined that the 4spd in my 68 is the M20.

The 4th gear on all 4 spds is 1:1. The 3rd gear on the M20 is 1.46 :1 , but on the M21 the 3rd gear is 1.28 :1.

So, with the car in 4th gear drive along nice and slow and get your rpm to 2000, and keep your speed steady. Not sure how fast that is, depending on your rear gear ratio... somewhere around 35, 40 mph? Doesn't matter, just make sure rpm is steady at 2000. Now downshift to 3rd gear while keeping your mph steady. Don't let the car's speed change, just increase the rpm in 3rd gear as much as you need to keep mph steady.

If rpm jumps up to 2560 in 3rd gear, you have an M-21 (2000 X 1.28 = 2560)

If rpm jumps up to 2960 in 3rd gear, you have an M-20 (2000 X 1.48 = 2960)

You might have to try it a few times, shifting back and forth between 3rd and 4th, but the difference between 2960 and 2560 is enough so that there should be no doubt as to which trans you have.

73 Vet

Go to: www.muncie4speed.net