# Muncie M-20 casting number help?



## mysons69 (Mar 3, 2012)

I’m hoping someone can help with Muncie identification. The car currently has a Saginaw in it, and that obviously has to go, so we’re going to have to start hunting for a Muncie. We’re trying to determine the correct casting numbers to look for to buy a Muncie M-20, which was the original trans for our 1969 GTO.

What would be the correct casting numbers? My research says we should have the following, based on info from Muncie 4 Speed Transmissions
Case 3925660	(1968-1970, I'm pretty confident this is correct)

Tailhousing	3846429	(1963-1965 Regular thick web, driver speedo, 27 spline per above source - VERY unsure if this is the tailhousing we need)
OR 3857584	(1966-1970, non-Pontiac?)

Side Cover	3884685 (or 3950306?)
From what I understand (which is limited, for sure!), the 3884685 cover has studs for the shifter arms, while the 3950306 cover uses bolts to mount the shifter arms. I have not been able to determine what triggered the use of either one, nor which is correct for a ’69 GTO.

Also, the speedometer mounts on the driver side, correct? The assembly manual seems to show it mounting to the driver’s side of the tranny, which then points to the 3846429 tailhousing, but all info I can find says that p/n was only used up till 1965. I’M CONFUSED! :confused Most Muncies I see for sale have the 3857584 tailhousing, but the speedo mounts on the passenger side with that extension.

I’ve looked through the 1969 Pontiac Service Manual, as well as the ’64-72 GTO Restoration Guide and Assembly manual, but I can’t find this info anywhere.
Can anyone help clear this up? Thanks.


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## mysons69 (Mar 3, 2012)

Nobody ?!?! I thought for sure some of you guys could answer this, or at least offer an opinion! No one has pictures of their original Muncie?

Anyway, regarding the tailhousings I mentioned above, I'm pretty sure the 3846429 must be correct, as it has the speedo on the driver's side (I seem to remember seeing a post saying that would be correct for a GTO), while the 3857584 has the speedo on passenger side.

We're not stuck on originality, but if we're going to buy a tranny, I figure it would be better to get a period and vehicle correct one if possible, with all other things being equal.

Am I correct in the assumption of driver-side speedo for the GTO? Has anyone used a passenger-side speedo? If so, what did you have to do to use it? I'm thinking the speedo cable won't be long enough to go all the way to the other side and still be able to mount straight in.

BTW, it's looking more and more that the 3846429 tail was also used on early Corvettes, though I'm basing this on internet research, which can be iffy, as we all know.

Thanks for any input.


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## ALKYGTO (Mar 29, 2010)

Thought for sure one of the resto guys would have popped in here.

I've got my Muncie on the shelf in the basement here but I need to dash off to work so i wont be able to look until tomorrow. If no one else on here has answered by then I'll post up the #'s on mine or confirm yours. 

What I can tell you is that M20 does have the spedo drive on the drivers side on all A body GM's and I would definitely use that rather than the pass side drive or have to transplant another tail shaft. Also the all have 10 spline input shafts. :cheers


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## rickm (Feb 8, 2012)

my '65 has a '69 m-20 with the passenger side speedo mount, that is incorrect for '65. i bought a general motors cable from ames part # R158LS. it measures 79" and threads on the back of the speedometer head. it reaches fine with no problems with kinking or operation. i cable tied it to my crossmember for stability. ames has a wide selection of gm cables .


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## rickm (Feb 8, 2012)

'63-'65 m-20 10 spline 24 tooth count. '66-'70 m-20 10 spline 21 tooth count. '70-'74 m-20 26 spline 21 tooth count. ( identifying muncie 4 speeds II by paul cangialosi )


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## mysons69 (Mar 3, 2012)

Almost all of the M-20's (and M-21's) I'm seeing have the 3857584 tail, which is passenger side speedo. Funny thing is, I've seen a few Chevelle posts where they say they think the 3846429 tail is from a Pontiac. 

Also, the passenger side tail speedo (3857584) seems to point somewhat down at an angle, while the driver side speedo looks as if it enters almost straight in. That's my take from the photos I've seen, not on any actual experience though.

Well, I'll keep trying to research this. FYI, while it seems almost everyone uses the data from 5speeds.com when spewing Muncie info, this site Muncie 4-speed has a little more info, and it's the ONLY place I've found so far that says the 3846429 was used from '66 to '70, rather than up to '65. If he is correct, then it all starts to make sense.

ALKYGTO, you're input would be GREATLY appreciated.

Thanks for the help, guys.


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