C1 - Differential

[Jim K.]

I'm looking into changing out my 3.70 posi to something even more highway friendly. I bought a non posi 3.36 diff and was hoping someone could tell me what I have, given the following numbers:
top: J196
Bott: 9725899
then a big "GM 10"
I'm told that the swap can be made from posi to non-posi, but mot the other way. The goal is to make sure I am happy with the gearing before investing in a "real" posi or even just a new ring and pinion. I do need to replace the seal going toard the drive shaft, so knowing what I have seems important. Thanks for the help!!!
Jim

[Joe L.]

Re: C1 - Differential

Jim-----

I believe that the casting number on the case is GM #3725899. From the information you provided, all that can be told is that you have a non-posi differential case which was probably cast on October 19, 1956. This case was not used in PRODUCTION after 1964 (57-62 for Corvettes and passenger cars; 63-64 for passenger cars only), so unless it was a SERVICE case cast in 1966, it was cast on the date mentioned above.

Nothing about the gear ratio can be discerned from any of the information which you provided. However, for 1957 (the model year that this case would have originally been used for) a 3.36:1 axle ratio was not used for any Corvette. 1957 passenger cars with Powerglide did use a 3.36:1 ratio so, assuming that you do have a 3.36:1 ratio, it's probably a passenger car unit that you have. That's ok, though, as the passenger car units and Corvette units were, basically, the same.

I appreciate your desire to use a lower numerical gear ratio for highway purposes. However, one thing to keep in mind here is this: the reason that higher numerical axle ratios were generally used in C1 Corvettes was that the 283 engine just didn't produce a lot of torque. So, 1st gear performance wouldn't be too good with a low numerical gear ratio. If you convert to a 3.36:1 gear ratio, you might not be satisfied with 1st gear performance, although your cruise will be more pleasant.

[Jim K.]

Re: C1 - Differential

Thanks Joe, I was hoping you would respond. All of what you said, I was expecting (except the 66 service replacement case thing), I knew there might be an issue with first gear, and that is why I wanted to test out a less expensive 3.36 non posi unit. If I like it and it works O.K., then I'll think about new gears or a new diff. posi unit (but I would hate to shell out 6, 7 or $800 on a posi unit that I hated!!!). On the companion flange end of things, is that seal something that I can find at the local Napa shop? Thanks for your help Joe!!!
Jim

[Mike E.]

Re: C1 - Differential

Flange seal (pinion seal) is as common as dirt--any parts store should have it. You can go back and forth between posi and non-posi without issues. The posi axles were a slightly different length than the non-posis, but I've been through many solid axles where an original posi had a non-posi installed, and vice-versa.
It's good to put a pinion (companion) flange seal in--the only solid-axles that I've seen that didn't have gear oil slung on the mufflers were the ones that didn't have mufflers. (Then it was only on the bottom of the body, etc.)

[Jim K.]

Re: C1 - Differential

Thanks Mike! If it works out then I'll be looking for 3:36 ring and pinion for my posi and I'll rebuild that, with new seal to keep my new muffelers clean ;-)