C-1 Differential Parts

[Gerry P.]

I'm looking for guidance in determining what non-posi differential gears/guts will fit in the Oct '56 differential housing I've located for my '57. From earlier postings, the 55-64 complete differentials are interchangable but I assume the inside parts may not all be compatible. As an fyi, I plan to install either 3:55 or 3:70 gears in the carrier.

Thanks
Gerry

[Terry D.]

Re: C-1 Differential Parts

Gerry
In a non posi with those gear ratios, anything from 55-64 will fit. The posi units are what's different. One unit will work with 3:70 gears and lower (Arma Steel C30743) and the other unit (ends in 751) uses gears 4:11 and up. Hope this helps
Terry

[Gerry P.]

Re: C-1 Differential Parts

Thanks Terry for the reply. I just wanted to make sure that before I started searching for missing parts, including spider gears, shafts, bearings and ring & pinon gears (or maybe a complete differential just for the parts), that I wouldn't end up with items which would not be compatible with my Oct '56 differential housing.

[Tom P.]

Re: C-1 Differential Parts

I've been rebuilding these rears for 40+yrs, and here is what is, and is not common and/or interchangeable.
The 55-64 pass car and 56-62 Vette rears share a LOT of commonality and interchangeability.
They ALLLLLLLLLLLLLLLLL have the same axle splines, thus, the assembled center sections are physically interchangeable between ANY 55-64 pass car/56-62 Vette axle housing.
ANY and ALL 57-64 guts will interchange (surely you know that ring and pinion gears are matched sets, so you can't use a pinion from one set with a ring gear from another set). The 55-56 guts will interchange, BUUUUUUUUUUUUUUT, the 55-6 and 57-64 guts do not exactly interchange with each other.
In the SAME gear ratio, ie 3.70, the 55-56 ring gears are THINNER than the 57-64 ring gears (pictured below). THUS, the ring gear flange of a 55-6 carrier is in a different position than it is on the 57-64 carrier. Also, the 55-6 carrier is NOT as beefy as the 57-64 carrier (pictured below).
If you have a BARE 55-6 center case, you can put 55-6 guts in it (R&P, carrier, etc), OR, you can put 57-64 guts in it, but you can't mix them. Also, you can put 55-6 guts in a 57-64 bare case----------------but why in the world would someone want to put thinner, weaker parts in a later case????????????
ALLLLLLLLLLLLLLLLLLLLLL 57-64 differentials were either 3-series or 4-series (both posi and non-posi). The 3.08-3.70 gears were used on 3-series differentials and 4.11-up gears were used on 4-series differentials.
Last, just for identification purposes, in addition to the casting number on center cases as positive identification, the 55-6 cases were NOT as beefy as the 57-64 cases.
This is a comparrison of a 55-6 case and the 57-64 thicker, stronger case.

55-6 (that's had a posi added)


Thicker, stronger 57-64 case


Thick 57-64 ring gear (left)-thinner 55-6 ring gear (right), BOTH are 3.70.


Thinner, lighter, weaker 55-6 differential carrier and 57-64 style (both are non-posi)






These are non-posi differentials, but the flange location will be the same no matter if it is posi or non-posi for the 3-series (left) and 4 series.
The reason for the difference in the position of the ring gear flange is because 4-series gears have a smaller pinion gear than the 3-series. Thus, the ring gear must be moved closer to the pinion for 4 and 5-series gears. That's why there is a 3 and a 4-series differential.


You can put either a 3 or 4-series ring gear on a 3-series carrier (use a spacer to put a 4-series ring gear on a 3-series carrier). BUT YOU CAN ONLY PUT A 4-SERIES GEAR ON A 4-SERIES CARRIER.
As I mentioned, BOTH posi and non-posi carriers (differential) come in 3 and 4-series versions, and they will fit into ANY 55-64 center case. If you have a 57-64 NON-POSI center case, and you want to convert it to posi, there is a gusset in the NON-posi case which must first be partially removed to make room for the posi unit (the genuine posi case does not have this gusset).

NON-posi case BEFORE partial removal of the above mentioned gusset.


After partial removal of the gusset.


You can see here how close the clearance is for a posi unit (and you can see the partially removed gusset).

[Gerry P.]

Re: C-1 Differential Parts

Thanks Tom - I was hoping you would comment. Your explanations are very helpful. One other question, can I assume that an Oct 10, '56 casting date will be for the '57 year? Sorry, I don't have my differential housing here close by so can't compare it with your pictures.
Gerry

[Tom P.]

Re: C-1 Differential Parts

Well, yes and no. You REALLY need to look at the casting number.
A 57 case should have 3725899. There was a 3743833, but that didn't come on the scene until later for posi rears (about Dec).
If it just happens to be 3707306, it's a 56 case. My 56 is 104 units from the end of 56 production, and my car was built Sep 7, 1956 (got that info from Jack Knudsen at the St. Louis plant during the 81 NCRS Natls). Thus, by extrapolation, the last 56 was probably built around Sep 14, 1956. Therefore, ANY rearend dated mid-Sep or later, SHOULD be a 57.

[Gerry P.]

Re: C-1 Differential Parts

Thanks Tom, I'll do the casting number check. I really appreciate your help on this - you're really a great source of info.
Gerry Phillips