Hi girls and guys, Flandy here with a request for help, regarding the trans in my 68.  Here go's, casting number on the pass. side is is 3088336, my question is m-20, m-21, and what years available on vettes?  Thanks-in-advance Phil

Hey, Phil...
The # you give is simply the casting # of the case. If you look on the machined surface, left side of the case,behind, and somewhat under the side cover(shift plate), you should see some stamped numbers/letters.It will start with the "prefix" num., followed by letter representing the last digit of the year, followed by two nums. for the day of the month, ending with the gear ratio suffix. Wheew!
Example:
P8A24** =(P)Muncie 4spd-(8)1968 year model-(A)Jan.-24- Gear ratio.
I don't have the early Muncie ratio charts here....maybe someone else here does.
A little bit of info, tho. The YEAR digit (8 in the example) represents the year model of the VEHICLE it is used in, NOT necessarily(?) the year the trans was assembled. Therefore, you could have a date code of January, 1968, but the trans is in a car assembled in December, 1968. We know the trans would not be sitting around the plant for 11 months, waiting to be used in a car. The Muncie plant simply stamped the model year that the trans was to be used in, not the year of production. Confusing, to be sure. There will also be a VIN # stamped in the same area of the trans.

Casting# for a 68 M-21 maincase should be3925660. The extension should be 3857584 and the side cover casting should be 3884685.

HEY PHIL, my 69 M21 close ratio has P9H11C  3088336, if its any help

Is the 3088336 stamped or cast? The numbers I looked up were casting #s

cast

Interesting, my  Corvette by the numbers book doesn't show that #.

I'll look again. I'd bet with 2 vettes with the same number, my book ain't right!

Thanks Jack!

     this site might help

 http://www.5speeds.com/muncie2.htm

 well daveB12 after reading your post I thought that a little strange also, so I put on my glasses this time and went under the vet to investigate.You're book was right, the number 3088336 is a GM patent number. The case casting number on my tranny is 3925660 with a serial number of P9H11C. Now let me say thank you, without your input I would never have gone under the vet to look at these numbers. Thus I would have been under the impression I had the M21 close ratio as was stated on the build sheet. I just found out I have an M22 rock crusher, imagine that. All documented M22 trannys used the 3925660 case, and the C at the end of the serial number confirm it. Guess there was a tranny change somewhere back when. JacksL7138758.8252199074

That's a hell of an upgrade! Almost like finding gold! A crusher is worth a lot more than an M21!