1968 Help

[Jim Mac]

Hi Everyone,

Is there anyway to Identify between a 1968 M20,M21 and M22 transmission? They all appear to have the same maincase# 3925660.
Was the M22 only found on L88 cars? I only see the last 4 digits of the vin on the transmission, should all of them be in place? Finally, Did 1968 BB cars come with a certain rear? Thanks In advance for any help. Jim

[Joe L.]

Re: 1968 Help

Jim-----

For 1968 the M-22 was available only with the L-88 engine. In fact, it was required with the L-88 engine and available with no others.

The main case is the same for all 1968 Muncie transmission variants. However, the M-22 version of the case for 1968 had the lower boss on the passenger side drilled and tapped for a drain plug. If the plug is there, though, it could possibly have been added. If it's not there, it likely means that the trans is not an M-22.

With the transmission out of the car, one can inspect the input shaft. M-22 had no circumferential rings machined around the input splines. M-20 had 2 such grooves and M-21 had 1 groove. If you find an input shaft with grooves, then you know it's an M-20 or an M-21. If you find one with no grooves it's either and M-22 or one of the other variants with a replaced input gear (many replacement input gears in days of old had no grooves on any of them).

There may be a small metal tag still attached to one of the lower side cover bolts. The part number on that tag can ID the transmission (assuming no one has removed or replaced the tag).

No suffix code on the transmission manufacturing code was used for 1968, so that can't be used to ID the transmission.

Usually, the VIN derivative on these transmissions is more than 4 characters long, but it's not the complete VIN number. There should be 2 sets of stampings (assuming that the transmission was originally installed in a vehicle and not a SERVICE unit). The manufacturing code should begin with a "P".

There were several difeernt rear end ratios used for 1968 big block Corvettes, depending upon which specific engine was installed, which specifc transmission was installed and whether or not an optional ratio was ordered for the specific drivetrain.