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I have a quick question. If a Muncie has an impute shaft with 2 rings, is it defiantly no doubt 100% a m20? Can a 2 ring impute shaft be put into a m21? See my old muncie in my elco sure as heck felt close ratio when I shifted but it has a 2 ring impute shaft and the case decodes as a M20. I swear it felt like a m21 could this have been changed in a rebuild at some time?

Thanks a lot guys

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Thomas (AKA Hot66ss)
66 Super Sport Now striped frame off Project
67 Malibu Getting there.
69 El camino Beater Mild 355, M21 4 speed (Daily Driver)
Video of me in the Elco smoking the tires
Aces Member 2380
Team Chevelle member #262
 

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Since Muncie parts can be interchanged during a rebuild the numbers or shaft rings may not tell you which trans it is. The best way to see what you have is to open the case and count the teeth on the input shaft gear.
M20 will have 24 teeth or less on some models
M21 has 26 teeth cut on about a 45 deg angle
M22 also has 26 teeth but will have almost straight cut teeth (about 20 deg. angle)
 

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It is definitely an M20. The grooves on the input shaft and corresponding grooves on the cluster gear were meant as an assembly aid when the transmissions were first built. The input/cluster gears are the only difference between an M20 and M21 and the grooves indicated to assemblers which cluster went with which input. There is a noticeable difference in the 3-4 shift RPM drop between an M20 and M21. One thing to keep in mind is that if an aftermarket input or main drive gear were installed there are probably no grooves on either gear. I have an OEM GM M21 replacement input gear (purchased from Chevrolet in a GM box) which has no grooves so even Chevrolet can confuse you!
 

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Thomas,
YOU ABSOLUTELY AND POSITIVELY CANNOT MATE AN M-20/M-21 INPUT AND/OR CLUSTER. They will NOT go together.

ANY (63-74) M-21 input and M-21 cluster can be mated to each other------------------BUT--------------63-65 M-20 inputs WILL ONLY mate with a 63-65 clusters and a 66-74 M-20 input will ONLY mate with a 66-74 M-20 cluster.
One additional thing. The 63-65 main cases used a 7/8in cluster shaft/cluster gear and the 66-74 main cases used a 1in cluster shaft/cluster gear. Therefore, you must use the appropriate gear/shaft in the corrosponding year(s) case. You CAN use a 1971-74 fine spline M-21 input in a 63 case because it will mate with a 63 cluster, but you CANNOT use ANY 66-74 M-20 input with a 63-65 M-20 cluster (totally different tooth patterns).

And just for information, in case anyone wants to know, You CAN install a 66-74 cluster/shaft (any gear ratio) into a 63-65 main case. It involves having the 7/8 hole bored out to 1in. THIS MUST BE DONE BY A VERY COMPETANT, EXPERIENCED MACHINIST WHO IS TOTALLY FAMILIAR WITH MUNCIE SHAFT HOLE DIMENTIONS!!!!!!! This little modification would allow someone with a matching number, early Muncie, to up grade to a 1in cluster and a fine spline input.
And then, while we are on this same subject, this same sort of mod can be done to a very early Borg-Warner T-10 and then install a complete late model Super T-10 gear set. For example, a matching number T-10 from a 58 Corvette (with thin, weak gears, can be up graded to Super T-10 strength).

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Tom Parsons
 
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