cdj46

I am trying to ID my 4 speed, I think it is a M22 however they only installed ~ 80 in 1968. Casting # is 3925660 and date code is P8T12. Corvette is a 68 427/390 HP.
Thanks for any help Chuck

sullyman56

There should be a letter after the number 12. I have a 68 similarly equipped and mine has the M20 wide ratio four speed. " A" is for the M20. "B" is for the close ratio M21 and "C" is for the M22. My guess would be yours is probably the M20. On mine the letter is difficult to see and required some cleaning.

Easy Mike

Is your tranny original? Your VIN derivative will be stamped on it if it is. Was your car originally an L-88? For 1968, the M-22s were available only on the L-88 cars. You couldn't get the M-22 with a 390hp big block.

cdj46

The trans has the vin stamped on it and it is right however it is an L-36.

Easy Mike

Yep, so that eliminates M-22 as a choice.

joewill

it was not until the model year 1969 that the A, B, or C was always stamped as a suffix to the assembly date code. before that, some did and some did not. since it is pretty sure you don't have a M22, and the maincase and extension housings were the same, you will just have to count the teeth in the gears to see what you have.. in 1968, the M20 and M21 were about 50-50.
if 1st gear seems to lug.. then you may have a M21... due to the higher ( lower numerically) gear.

cdj46

I looked in black book it told me L88 80 made M22 80 installed, so no chance I have a M22. Thanks to all for the info. Also there is no A,B,or C after the date code. Most important to me is the correct vin on it.