The Stovebolt Site Forums The Shop Area Driveline M21 Muncie 4-Speed Broken Reverse Shift Stud

Looking for a little help from the Muncie transmission guru’s.

The M21 Muncie 4-speed (not oem to the truck) in my ’65 C10 has a broken reverse shift stud. Is it possible to drill and tap the reverse shaft or is it a hardened part?:
(1963-1965 vintage M21: Main case 3851325, Tail Housing 3846429, Side Cover 3831707)

http://i645.photobucket.com/albums/...%20Transmission/DSCF1810_zpsf7f3c46b.jpg

http://i645.photobucket.com/albums/...%20Transmission/DSCF1808_zps21ad2b2b.jpg

I have a spare M-21 which is a newer vintage M21 (‘68-’70). I bought it about 15 years ago, don’t know much about its history. Funny thing is it also has the side stud style shift arms, except for reverse, which has a bolt in it. Does this seem "factory correct" or is this more likely to be a modification from a previous owner ?
(68-70 vintage M21: Main case 3925660, Tail Housing 3857584, Side Cover 3950306)

http://i645.photobucket.com/albums/...%20Transmission/DSCF1818_zps3b449d2c.jpg

http://i645.photobucket.com/albums/...%20Transmission/DSCF1817_zps38a4f93c.jpg

I'm definatley not a Muncie Guru, but have had a few of them over the years, and have one in my '55 pickup now. I'm wondering if the stud is an integral part of the shift shaft or a screw in stud? The one in the reverse shaft in your newer one has either been replaced or the stud was unscrewed and a bolt substituted. If the broken stud can be unscrewed out, you are home free. I would think that the shaft with the broken stud can be drilled and tapped, but the tranny would most probably need to be removed from the truck and disassembled? Wish I could be of more help, maybe a Muncie Guru will!

They either came with the stud or bolt, never a mix of both. I think that drilling it out would be difficult even if done with a drill press. I would pull the tail shaft off the transmission then get a better look at the broken bolt/stud.

Thanks for your input guys.

I am being told by fellas that sell and rebuild these that:

* It's not a screw in stud, the 5/16-18 threads are part of the shaft, and the shaft is hardened steel.

* The tail housing cannot be put back onto the transmission while the transmission is in the truck, due to a "spring and ball" arrangement inside the tail housing.

At least I'm learning alot

Gotta be pretty hard to break that stud, guess maybe if the nut worked loose?

Agreed. It's been cracked for a long time, it has heavy rust in the cracked area.

This trans was made in 1965. They changed the design in '69 from a 5/16" UNC thread to a 3/8" tapped hole... doesn't sound like much, but it increases the strength by 44%.

I was able to repair the busted threaded portion of the reverse shaft discussed above by drilling and tapping the shaft while the transmission was still in the truck. I also confirmed what the Muncie guru's were telling me, the original OEM shaft design was not a screw in stud.

Numerous sources told me that the shaft is hardened, and I believe it is, but not very hard. Drilling was fairly easy, tapping was a bit unnerving, but successful nonetheless.

The repair was made easier by the exhaust routing I did quite a while ago. By simply unbolting the exhaust from the headers and hangers I was able to access the transmission easily.

http://i645.photobucket.com/albums/...%20Transmission/DSCF1918_zpsc3f69e74.jpg

http://i645.photobucket.com/albums/...%20Transmission/DSCF1920_zps6cb5b006.jpg