Hoping someone on the forum can help me... I have a 1965 Corvette coupe I am restoring which is largely original with numbers matching engine. The factory original transmission was replaced at some point
with a unit from a '67 Corvette. I am working to put the car back into its original configuration and would like to source a correct spec 1965 4 speed transmission for the car.

Attached are photos of the data plates... my question: is there a way to determine the correct transmission for the car based on this information? I'm not clear if a close ratio or wide ration transmission is correct
for the car.

Thanks in advance for any help!

 

Neither of those plates will reveal the original transmission. Instead look at (and post) the engine pad to determine the engine assembly code. That will give you a clue.

 

Thanks Jim... will do

 

Thanks Jim... will do

 

The HH suffix on your engine stamp would indicate an L76 / 365HP engine. So very likely the transmission was an M 21 Muncie. Also, I believe the casting number on the transmission should be 3851325.

Ted

 

You might want to talk to this Member:

https://www.corvetteforum.com/forums...re-muncie.html

 

Agree with this. My L76/365HP '65 built 389 units before the OP's 65 has the matching drivetrain. It's a close ratio Muncie 4-speed, aka the M21.

 

Curious...my Black Book does not show M21 as an option until 1966. Am I missing something?

 

I don't think that you are 'missing something.' I think that the official factory use of M21 to denote the close ratio Muncie 4 speed was more or less 'official' but someone that tracks these things can clarify this. That's why I used the term 'aka.'

 

Are you missing something?
Yes and no.
For the 1963-65 model years, THERE WAS ONLY ONE OPTION FOR THE 4sp. It was M20, PERIOD.
The FACTORY------NOT THE CUSTOMER-------determined if the Muncie was a wide or close ratio. This determination by the factory was dependent on engine option and rearend gear ratio. LATER TO BE REFERRED TO AS M20 OR M21 beginning in 1966
Then, beginning with the 1966 model year, the 4sp options became M20, M21, M22.
And, speaking of M22, A VERY SMALL handful of M22 Muncies were installed later in the 65 model year. These were installed behind the very hi-perf engines, such as the solid lifter SB and the 425hp/396. Information and documentation on the few 65 M22s which were installed is very sketchy.

 

DZAuto is correct the factory determined the transmission used based on the rear differential ratio. For example with the HH engine code, if your rear ratio is 3:36 the car would have received the M20, if car had 3:70 rear then the factory would have installed M21. Identify the differential ratio your car has from the date and code letters stamped into the differential's bottom ID pad. To confirm the ratio you will need to have the rear wheels off the ground and count the number of revolutions the drive shaft turns to one revolution of the rear wheels.

 

So...in 1965, what did they put behind the L79? M20 or M21?

 

Again, it would depend on rearend gear ratio.
With a high gear rearend ratio, such as 3.08, 3.36. it most likely got a wide ratio Muncie NOT an M20 specifically.
If it got a low gear ratio, such as a 3.70, 4,11, it normally got a close ratio Muncie, NOT an M21.
Back then, CLOSE and WIDE ratio is the way Muncies were referred to. NOT, repeat, NOT M20 or M21.
I don't know how to explain it any better.

 

Maybe the 1965 Chevrolet power team listing will help clear this up.

 

Below are the born with specs for the diff and transmission as puled from the stampings. From your description, it sounds like mine has a wide ratio - but I never would guess it by the feel of 1st thru 3rd...
Muncie M2? transmission – 3851325 stamped P05 12 (May 12th 1965) SN S117792
Differential – 3876476-N stamped (A)M5 14 05 (May 14th 1965, 3.36, posi (rebuilt as 3.55 posi)

 

For 63-68 models---------IF THE TRANSMISSION IS IN THE CAR-----------there is NOTHING on the outside of the transmission which indicates wide or close ratio.
For the same years, ALLLLLLLLLLLLLLLLL Muncies used the same case, regardless of wide/close ratio or M20, M21, M22. Just one example, ALL 1967 Muncies (M20-M21-M22) used the same case with the same casting number, 3885010. Same for tail housings and same for side covers.
Thus, pull the transmission and look at the input shaft-------------------------MAYBE, MAYBE, MAYBE! FROM THE FACTORY, 63-65 wide ration Muncie input shafts had NO grooves on the input shaft. Close ratio had ONE groove. REMEMBER, WE'RE TALKING ORIGINAL FACTORY CONFIGURATION.
The 1966-74 M20 (wide ratio) Muncies had 2 grooves on the input shaft.
The 66-74 M21 Muncies (close ratio) had 1 groove on the input shaft.
The 66-74 M22 Muncies (heavy duty/rock crusher) had NO grooves.
Again, the above were FACTORY configuration.
BUT, if the gears were busted, or teeth broken off, A REPLACEMENT INPUT SHAFT USUALLY did not have any grooves.
Thus, that finally brings us to the question "How do you SURE FIRE determine what you actually have inside your Muncie case. Super simple, pull the side cover and check the tooth count of the gears.+



Frequently, unknowledgeable (or unscrupulous) people will try to pass off an M20 or M21 as an M22. If it has 1 or 2 grooves, run forest, run!
And, if it has no grooves, that DOES NOT necessarily indicate that it is an M22. It could be a rebuilt M20 or M21 that got a replacement input shaft.
AGAIN, the ONLY way to tell is remove the side cover and look at the gears. The angle of the teeth in an M22 are at a lesser angle than M20/M21 teeth, as below.

 

I am always impressed with the knowledge (and the knowledge sharing) of the folks on the forum.
So, in (early) 1963 it was a BW 4spd to change later to Muncie with no named identity per se.
I find it funny/not funny the factory determined what trans (wide/close) the customer wanted based on engine HP.
My 66 has a 300HP and 4 speed. It dops about 600 rpm per gear, thus I assumed it was a close ratio. I guess I need to roll my ol' fat body under the car to see what numbers are on it to see if it was born with it.

 

And AGAIN. If the transmission is in the car, for 63-68 models, there is NOTHING on the outside of the case that identifies if it is a close or wide ratio.
Remember, on ALLLLLLLLLLLLLLLLLLLL 63-65 Muncies, they were only M20.

 

By jove, I think I've got it.

 

Thanks DZ! Consider us well schooled on this topic now. Amazing amount of clarity shared here!