I got this Muncie 4-speed the other day. The fellow I got it from got it for free many years ago and never put it in anything - it's just been sitting in his garage. He believes it's an M-21 out of a '68 Oldsmobile, though he can't say for sure. I've searched the internet but I am finding conflicting data. Can anyone help me identify which Muncie model this is? I'm fairly sure it's not an M-22, as the input shaft has two ring grooves on mine, while the M-22 is supposed to have none. I've read that the M-20 had two grooves and M-21 had one... I've also read the opposite! Also the gear teeth angle aren't as straight as an M-22s. The M-22s supposedly 20 degrees, while M-20/21 are 45 degrees. I'm hoping there's some old timers or Muncie experts out there who can help me identify whether this is an M-20 or an M-21. Thanks for any help you can provide! Note: Yep, I know the forward lever on the side-cover is upside down. It's since been corrected.
This should help. FIRST DESIGN MUNCIE The first design was from 1963 to 1967 and is considered the weakest of the bunch. The cluster shaft is 3/4 inch dia., the selector plate has an internally pressed stud. Input shaft is 10-spline and selector shafts are outfitted with 5/16th inch studs. Example: Typical date code0503N P-muncie 05-May 03-Third day. D/N-Day or night shift(not always coded) There is no year designation on these transmissions. MONTH 01-January 02-February 03-March 04-April 05-May 06-June 07-July 08-August 09-September 10-October 11-November 12-December THE SECOND DESIGN The second design muncie (1967-1968) increased the cluster shaft diameter 7/8ths inch and included the second design selector(side) cover with the larger diameter pin externally pressed in to prevent it from shaking loose from the case. Selector shafts studs increased to 3/8ths inch. The 10-spline input shaft remained. Typical date code: P7T02 P-muncie 7-1967 T-December 02-second day MONTH: A-Jan. B-Feb. C-Mar. D-April E-May H-June K-July M-Aug. P-Sept. R-Oct. S-Nov. T-Dec. THE THIRD DESIGN(1968-1973) Due to an increasing number of selector shafts breaking off, the only change made in 1969 was to alter the selector shafts by drilling and tapping them to accept a 3/8ths bolt instead of studded shaft accepting a nut. Subsequent component changes made in 1970 changed the input shaft to a 26-spline(fine) and increased output shaft diameter to accept a T-400 trans. yoke. The final incarnation of the muncie lasted until 1973 when the transmission was taken out of service. 1969-1973 Typical date code: P9B08C P-muncie 9-1969 B-February 08-Eighth day C-M22 MODEL IDENTIFICATION(1969-1972) A-M20 B-M21 C-M22 Note: not always stamped Originally, identifying the transmissions by external markings was simple. The M20(wide ratio) trans. had two grooves cut into the 10-spline input shaft and no fill plug in the case, while the M21(close ratio) had a single groove on the 10-spline input shaft and no fill plug on the case. The M22 had no grooves on the input shaft and was the only transmission to be equipped with a drain and fill plug. Unfortunately, over the years, replacement cases and gears offered by Chevrolet and aftermarket manufacturers did not follow the G.M. assembly line identification system, so trying to use this method to identify a transmission is almost impossible. To determine close ratio(2.20:1 first gear) or wide ratio(2.54:1 first gear), mark the input shaft and output shaft, engage first gear and spin the input shaft and count how many revolutions the input shaft turns to make one revolution with the output shaft. Just less than 2 1/4 turns is a close ratio, and a little more than 2 1/2 turns is a wide ratio. M22 transmissions can only be identified removing the selector cover(side cover) and visually identifying the straight cut gears-all other muncie transmissions had conical cut(angle) gears. The most well known Muncie transmissions are the M20(wide ratio), M21(close ratio), and the M22(rock crusher). Although not mentioned much, there was also an M23 transmission that was used, beginning in 1970. The M23 is recognized by a 26-spline(fine) input shaft and a large output shaft that requires a T-400 driveshaft yoke. Although some claim that the M23 is a 'rock crusher', it is indicated that this box was assembled in both wide and close ratio components and also with straight cut gears common to the rock crusher (M22) transmission. Looks like you have a second design-feb. 28th, 1968, M20 or M21. Definitely not a rock crusher(no straight cut gears). The "P8B28E" is the number you need to ID your muncie.
Thanks for the help fellas. It's been pretty much confirmed it's an M-20. Now for another couple question. How come the M-21 & M-22 were used on the high-performance stuff with high gearing, while the M-20 seems to have been in the shadow? Is it not able to handle the same amount of stress the other two are? Would the wider gear ratios on the M-20 be better for acceleration? Are there any merits of using an M-20 over the other models?
The M20 is a wide ratio and was the standard GM 4 speed of the time. M21 (close ratio) and M22(close ratio heavy duty) were options M21 and M22 have the same ratios but the M22 has straighter cut gears to help prevent end loading thats the only difference between the M22 and M21. the 2.20 first gear is not ideal for street driving unless you have a fairly low gear in the rear end like 4.11 or so. I prefer the M20 for street driving
I have a trans I believe to be a 3 spd with markings GM 23 3952845 L210. It's connected to a mid 70's Buick 350 and has a floor shifter. Thanks. Any ideas who made it. any info would be helpfull
Before you try to run this unit I would at least put a rebuild kit in it. If you look at the brass syncro ring on the input shaft you can see it is almost touching the gear teeth. You can buy a bearing and seal kit for about $120 if you shop around. Also make sure to flip the front shift arm to the top or 3rd and 4th will be reversed on your trans.
Good stuff but I'm curious where you got this info because it's wrong. 63-65 had a 7/8" cluster gear shaft & 66 & later had a 1" shaft. They never used a 3/4" shaft in any versions. I used to rebuild Muncie & T10s for a local Corvette shop and have done probably close to 100.
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