. . Hello again, , Got the new fan and the harmonic balancer (and spacer) on and timing set. . Had the car up on the lift checking the throw-out bearing play when I ended up at the rear end. .
. . This is a '62 340hp. . Gear ratio is supposed to be 3.55 positrac from what I can tell. . Well it is definitely 3.55 but not posi. . Where are the differential codes on the pumpkin? ?
. . Second question. . My speedo is off. . Checked my odometer with the mile markers and I get 9 1/2 tenths for each of their miles. . What speedo gears should be in the transmission for this 3.55 rear end? ?

 

I'd bet you'll find everything you want and need to know here:

https://gmheritagecenter.com/docs/gm...t-Corvette.pdf

 

. . This is why I get confused. . Different links contain different information. . This one lists code CA which is 3.36 and CG which is 3.70 but mine is 3.55 and there are other possibilities. .
. . My casting number does not match what a '62 should be, posi (3789812) or non posi (3725899) . .
. . My casting number is - 3707306 with D46 above it. . And the code is - BB412X . . Any ideas where this came from? ?

 

56 differential

chev and vette codes for the same year differ.

stamped info on forward face of diff on pass side, barely legible, look close

 

. . Ok, Date and casting number say it was originally in a 1956 of some kind. . BB says it was originally 3.36 gears. . Not now. .
. . So what does the 412X mean? ?

 

The 306 housing is a ONE YEAR ONLY for 56. PERIOD!
The D46 is the CASTING date, Apr 4, 1956.

READ THE FOLLOWING CAREFULLY.
There was NO 3.36 gear ratio in 1956. PERIOD!
The BB412X translate to 3.55 Powerglide in a PASSENGER CAR. and the 412X translates to an ASSEMBLY date of Apr 12. The stamped code DOES NOT contain the year because that is already in the CASTING date of D46. I'm not sure what the X translates to on a rearend.
Thus, bottom line, the rearend in your 62 Vette has been transplanted from a 1956 pass car that had a Powerglide tranny (a previous owner probably busted the original posi rear).

 

. . Yup, Starting to see that. . Had looked at one Impala site earlier that claims the BB code can be found from '56 thru '64 and that they are all 3.36 geared. . Not finding anyone who agrees with that. . . .
. . Next question. . I located a '62 812 differential that has 4.56 gears in it right now. . Will that 4.56 carrier take the 3.55 ring gear or are they like the later models, 3 series or 4 series. . ? ?

 

ANY differential (posi or non-posi) can be installed in an outer case.
BUUUUUUUUUUUT, ONLY a 4-series (or 5-series) ring gear can be installed on a 4-series carrier (differential). You CANNOT, repeat, CANNOT use a 3-series ring gear on a 4-series carrier (regardless of posi or non-posi).
The 812 case is a 62-64 posi case (has the big P), and you can put ANYTHING inside it (the posi case was used for BOTH 3- and 4-series ratios). It's the carrier itself (you know, the posi unit) that is different.
SOOOOOOOOOOOOOO, if you want a 3.55 posi rear, you can use the 812 case, but you will have to locate a 3-series posi unit. Now, as I mentioned, EVERYONE who has a 3-series posi rearend for sale, thinks it is worth gold. In my opinion, the best alternative to that is to buy and install an Eaton or a Nitro Power Lock posi unit and install it in either a posi or non-posi case.
DO NOT, DO NOT, DO, NOT, DO NOT, DO NOT USE THAT 56 306 CASE!!!!!!!!!!! The 55-56 rears were thinner, lighter, weaker than the 57-64 rears. If you spend a bunch of money converting the 306 case into a posi, you will probably bust it soon.
The 57-64 NON-posi case has a gusset inside which has to be partially removed so that a posi unit can be installed inside a NON-posi case. I did an article a few years ago demonstrating how to do this-------------BUUUUUUUUUUUUT, Photobucket has hijacked everyone's pictures which have been posted and they want a ransom ($400/yr) to allow 3rd party posting of pictures. Thus, the pictures no longer appear on my article!!! If you are interested, I can email you a PDF version of the article.
Below are 2 pictures of a before and after removal of the gusset that I mentioned.








That gusset is NOT, repeat, NOT in a posi case, as seen below.




This picture shows very clearly why a portion of the gusset in the NON-posi case has to be removed, because the posi unit fits VERY CLOSE to the wall of the case.




Oh ya, I forgot to clarify earlier. In 55-56 the BB ratio code was for a 3.55, but in 57 the BB code changed to 3.36.

UNFORTUNATELY, I don't know much about these early Chevy/Vette rearends because I've only been building them for almost 50yrs.

 

. . Thank you for the information. . Being new to a particular subject makes it hard to tell those who know what they are talking about form those who wish they did. .
. . While the car is not totally original it is pretty close. . It is my intention to put an 812 case with the correct gears back in it as that is a pretty major original part. . Whether I go with a posi I don't know. . It only gets driven around town or maybe down to Denver and then only on clear days. .
. . And yes some people think that if they have it is gold. . I'm seeing these differentials going for as much as $1,600 used. . While I don't want junk, I do want quality at a reasonable price. .

 

It's your car, it's your money, you can do as you wish. The 812 was a posi case (all posi cases have the big P cast next to the casting number).
Posi was still an option in 62. The casting number on a NON-posi case was 3725899, and was used on pass cars through 64. If you could locate a 1962 899 rear from a pass car (should be pretty cheap), IN GOOD CONDITION, that would be a good start for cheap. MOST 62 pass car rears were either 3.36 or 3.08. If you wanted to go posi, then you could install an Eaton or Nitro posi unit.


Here is an 899 (non-posi) case and an 812 posi case (with the big P).