1968 Corvette 4-Speed #'s

[Joseph M.]

Hello,

I recently purchased a matching number, 1968 Corvette 427/390HP 4-Speed, T-Top car. I would like to verfiy whether or not this car has a M-21 close ratio trans in it. On the side case I found a metal plate at the bottom with the numbers 3915090 on it. Also, I see a "S" stampled on the forward lower side of the case behind the bell housing. In addition, the casting # above the fill plug (no drain plug) is 3925660.

Thanks!
Joe

[Richard F.]

Re: 1968 Corvette 4-Speed #'s

AIM says that it's an M21 trans mated to a an L79 engine (327/350)

[Wayne M.]

Caution in interpretation of trans tags

Corvette did NOT make different 4-sp transmissions for big vs. small blocks. They DID, however, differentiate (by different metal tags bolted to the bottom forward of the shifter sidecover) whether the trans was wide or close ratio COUPLED WITH whether the trans had a speedo sender gear (on output shaft) suitable for 3.08, 3.36,3.55 OR 3.70, 4.11 final drive ratios. [4.56 was an additional case].

If the '68 AIM is anything like the C2 counterparts, it is extremely difficult and inconclusive trying to determine the engine horsepower from the trans part #'s shown in M20 and G81 sections. The Corvette repro catalogs listing of repop tags for specific engines is riddled with errors. Wide ratio trannys were always speedo-geared for the 3.08 to 3.55 family of differentials. Close ratio boxes could go either way.

If that "S" is facing forward, it's probably just an inspector's stamp. Have you verified if your car's VIN derivative and trans assembly date code are stamped vertically on the wide pad to the left of the filler plug ?

Finally, you can tell wide vs. close with the 3-to-4 shift and the number of rev's in each gear at the same speed. (1.46 times different [wide] vs. 1.27 close)

[Joseph M.]

Re: 1968 Corvette 4-Speed #'s

At a closer inspection of the Muncie 4-Speed, I found that the VIN code that is stamped vertically on the passenger side of the case matches what is stamped into the matching # 427/390HP engine and matches the VIN on the windshield pillar. Also, the other code that I found on the trans (P8K08) lines of with build date on the Paint code plate on the driver's door frame so it looks like I my trans numbers match the engine and body after all. :-)

P= Muncie
8= year
k= july
08= 8th day of month