What does this part # refer to.....

[Jason Conn]

The tag on the side cover of my '68 M22 reads...3915090. What exactly does this number refer too? Is it the part number for the whole trans? or the side cover?

Thanks, Jason

[Joe L.]

Re: What does this part # refer to.....

Jason-----

It's the part number for the complete transmission assembly. However, GM #3915090 is not the part number for an M-22. It's the PRODUCTION part number for an M-21 originally used with L-79 engines on 1968 Corvettes.

[Jason Conn]

Re: What does this part # refer to.....

OK....so my next obvious question is what tag should it have if an M22?

The stamp code on the side is P8C19c. Doesn't the 'c' designate M22?

a = M20
b = M21
c = M22

Thanks.

[Joe L.]

Re: What does this part # refer to.....

Jason----

For 1968 Muncie transmissions no suffix code was used so I don't understand how you have such a code on yours. Unless, of course, someone added it to the sequence. In any event, the M-22 for 1968 PRODUCTION was known as GM #3915093. For 1968, only 80 of these were installed with L-88 engines only.

[Jason Conn]

Re: What does this part # refer to.....

Joe,

I hear ya'. I have not read that about not stamping 68 cases. In all of the books I have for reference it does not state that in any of them. Why would the car's rearend be coded FB (HD 4.11) and M21 tranny?

Any other characteristics that would define if this was an M21 or M22? Any other part numbers? I have the tranny out of the car.

Thanks, Jason

[Joe L.]

Re: What does this part # refer to.....

Jason-------

The suffix code stamping which denoted the Muncie variant did not begin until October, 1969. So, even very early 1969 models did not have it. As far as I know, no 1968s or earlier did.

For 1968, your transmission case should be a "660" casting. For this series castings, only M-22s received the transmission drain plug. However, this feature could be added to a non-M-22 "660" case quite easily by a previous owner. If your trans does not have it, though, then it was not originally an M-22.

The best way to confirm which Muncie variant that it is, especially if you have the transmission out of the car, is to simply remove the sidecover and look at the gears. That will tell all tales. If you don't know how to identify M-20/M-21 gears from M-22 gears simply e-mail me a picture or post a picture and I'll be able to tell you on sight.

The HD 4.11:1 gearset (code "FB") was available with the L-71 engine and close ratio M-21 transmission, not just the L-88. A non-HD 4.11 (for which a PRODUCTION line substitution for the HD version is quite possible) was also available with the L-79 with M-21. That's the one that the transmission you have was supposed to be installed in.

[Jason Conn]

Re: What does this part # refer to.....

My link didn't go thru, that's why I deleted.

It looks as if you are right again, Joe. If I'm not mistaken, the M22's gears are lower pitched and a bit thicker at the edge. Also they are darker looking from the heat treating?? Also, the case has a side drain plug, and is stamped #660. Do you concur it is an M21?

PS How the heck do I get the side cover back on? I hope GM sells the gasket for the side cover. Thanks Joe as always.


)]Tranny cover removed[/URL]

[Jason Conn]

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[Jason Conn]

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[Jason Conn]

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[Jason Conn]

Re: What does this part # refer to.....

I think I got it!!

Bare with me....:)


Tranny case cover removed

[Joe L.]

Re: What does this part # refer to.....

Jason-----

Well, the drain plug makes me wonder about it if it's a '660' case. However, as I mentioned, a drain plug could have been added. If I could see the gears I could tell for sure.

The side cover can be a little tricky to install, but if you play with it a little you'll get it. It's not really difficult, at all. Putting the transmission in a certain forward gear makes it easier, but I've forgotten which gear, at the moment. Vaguely, it seems like it might be 3rd gear.

The gasket is still available from GM under GM #10140562 for a GM list price of $4.55. It's also available as part of a complete transmission gasket kit of GM #3917763 which currently lists for $13.80.

[John H.]

Re: What does this part # refer to.....

Jason -

It's an M20 or M21. 20/21 had a 45-degree helix angle on the gears (as your photo shows) - the M22 had about a 20-degree helix angle.

[Jason Conn]

Re: What does this part # refer to.....

Joe, open my last post, I finally got my link to work.
But now I think CF is down again.

[Joe L.]

Re: What does this part # refer to.....

Jason------

Yes, I see it now. And, I agree with John; the gears are M-20 or 21. Given the part number on the tag which you posted, it's likely the M-21 which it's supposed to be. By the way, is there 1 circumferential groove around the middle of the splines on the input shaft? If so, that would further corroborate its M-21 ID.

Also, installing the cover will be much easier if you remove the shifter shafts and shifter forks which I notice are still mounted in your transmission. First, install the shafts in the cover. Then, install the forks into the shafts. Lastly, install the complete assembly on the transmission, aligning the forks with the synchro hubs.

Also, you might inspect the forks for wear. The sides of the forks which engage the synchro hubs may show wear. If the wear is severe, the forks should be replaced. The forks are still available from GM under GM #3831717. They're expensive at a current GM list price of $88.50/each. But, if you need them, you need them. I can't see the important section of the forks, but the rest of the trans looks to be in pretty decent shape from what I can see. The all-important synchro teeth on the GEARS look ok.