67 Transmission Tag

[Jeff B.]

I have looked thru the main and archived threads for clarification, but am more confused than ever. Perhaps someone can answer this question. I have owned this 67, 427-390 w/air for almost 40 years. Car was built late Jan 1967, #89xx. Bought it in 1978 when it was just a "used car" that the owner could not afford the price of gas for it. Drivetrain is totally unmolested and original with AZ (3.55 posi) rearend. The transmission tag reads 3880853. Does this tag indicate whether the trans is an M20 or M21? Or what story does it tell?
Any info greatly appreciated.

[Joe R.]

Re: 67 Transmission Tag

Looks like you have a transmission tag 3880853 from a 1966 M21 close ratio for a 350HP car.

Or a 1967 Corvette M21 close ratio for a 350HP Car.

A correct tag for your car if the engine and HP is correct would be a 3880855.

Is your transmission assembly date and SN correct for a 1967?

JR

[Jeff B.]

Re: 67 Transmission Tag

Joe:
Thanks for your quick response. This is part of my confusion. I have the pertinent numbers written down, but not with me right now. However, the vehicle vin is stamped on the case and does match. I will double check the tag it see if the last number is indeed a 3, or maybe a 5. Do you see any problem with removing the bolt that holds the tag on, to remove tag?

[Dan A.]

Re: 67 Transmission Tag

Might be interesting to see what the differential code indicates.

[Jeff B.]

Re: 67 Transmission Tag

Car vin is 8902, built F23 (Jan 23). Trans is stamped 7S108902 P7S23. Rear end reads: 12-29-66 AZ.
Engine stamped T0113IL 7108902. Carburetor 3811 Dated 6A2. I believe this car to be a true unmolested survivor.

[Joe R.]

Re: 67 Transmission Tag

Jeff,

In other words everything is correct except that transmission tag. Probably left over from the 66 build and never used until your 67 came down the assembly line. I would change the tag and go on down the road.

JR

[Jeff B.]

Re: 67 Transmission Tag

JR
Thanks for your insight. Since this 853 tag may be suspect for the transmission, is there another way to determine if it is an M20 or M21?
Other than 1-3 gear ratio differences, is there any difference in their strength.
Jeff

[Joe L.]

Re: 67 Transmission Tag

JR
Thanks for your insight. Since this 853 tag may be suspect for the transmission, is there another way to determine if it is an M20 or M21?
Other than 1-3 gear ratio differences, is there any difference in their strength.
Jeff

Jeff------


Regardless of what it says or implies in the AIM, there were only 2 differences for any Muncie 4 speed installed in a 1967 Corvette. First is the variant (M-20,M-21, or-M-22). Second is the installed speedometer DRIVE gear. The required speedometer DRIVE gear is dependent upon the rear gear ratio. So, for any given rear gear ratio and Muncie variant, a transmission for any engine application would be exactly the same.

Your car has a 3.55:1 rear gear ratio. As such, the only 4 speed originally available was an M-21. Your transmission, GM #3880853, is an M-21. It includes a GM #3708145 speedometer DRIVE gear which is the correct DRIVE gear for Corvettes with 3.55 rear gear ratio. So, GM #3880853 is the correct transmission part number for your application. Given this fact and the fact that the correct VIN derivative is embossed, I have no doubt it is original to your car.

There is no difference in strength between an M-20 and M-21. There is a difference in strength between either of these and an M-22.

By the way, if you want to further confirm, simply remove the transmission DRIVEN gear. It should be a GREEN, 22 tooth gear.

[David B.]

Re: 67 Transmission Tag

From the Trans & Extension Assembly Chart dated 1-26-67 Tag 3880853 indicates the trans assy. internals are exactly the same as 3880855 with the only difference/exception being (as Joe indicated) the speedometer drive gear is 3708145. The drive gear in 3880855 is 3708144. Trans tags are super important since it tells everything inside the assembly -- Lucky you still have it attached! Really surprized after 40 years someone would suggest changing since it does not match NCRS data.
If you want to know what part #s are inside your trans send me an Email.

[Jeff B.]

Re: 67 Transmission Tag

Many thanks to all who have responded. I did check the driven gear, and it, indeed, is green with 22 teeth. Since one cannot differentiate between a M20 or M21 while installed in the car, I gather mine is the M21, since M20s were not installed with a 3.55 ratio.

[Joe L.]

Re: 67 Transmission Tag

Many thanks to all who have responded. I did check the driven gear, and it, indeed, is green with 22 teeth. Since one cannot differentiate between a M20 or M21 while installed in the car, I gather mine is the M21, since M20s were not installed with a 3.55 ratio.

Jeff------


Without a doubt. Plus, no M-20 was ever originally built with a GM #3708145 speedometer DRIVE gear. The fact that you have the 22 tooth GREEN gear means that you must have the 3708145 DRIVE gear installed.

[Joe R.]

Re: 67 Transmission Tag

Bartush,

When you are playing their game you play by their rules or stay at home.

I agree that is the original trans installed by GM when the 67 was made but as I say above.

JR