1963 Corvette Correct Differential Designation.

[Henry S.]

I have a 1963 C-2 Coupe, with a 327/340 hp engine and 4 speed transmission. Per the NCRS Shipping Data Report, the GM official production date of the car was May 5, 1963. Based upon casting and stamped information, the differential is from a 1976 Corvette, 3:08 ratio, with positraction.
I am seeking to replace the differential with one that is casting number, casting date, and assembly date consistent with the car's production.
I have located a differential for sale with a Feb 2, 1963 casting date, and a stamped differential ratio of CB, which, according to my NCRS 1953-1967 Corvette Specifications Guide translates into a 3.36:1 Positraction Manual Transmission or Powerglide Transmission.
My questions:
a) What is an acceptable date range from the time the differential housing was cast , differential was assembled and the car was produced. ?
b) The CB designation as described in the NCRS Specifications Guide identifies "Manual Transmission or Powerglide. There are separate designations (CC,CD,CF,CJ), for Positraction 4 Speed transmission. Does this mean the Manual Transmission identified in the CB, is ONLY the 3 speed (standard) transmisson, and does not apply to manual 4 speed transmissions.
c) Did 1963 327/340 hp cars have to be ordered with 4peed transmissions and or positraction, or could one have gotten a 3 speed manual, non positraction car with a 327/340 hp engine ?
Thank you for your wisdom.
Hank Sarno
NCRS# 22026

[Duke W.]

Re: 1963 Corvette Correct Differential Designation.

You should understand that the early C2 axles were of inferior design and manufacturing quality. Both the Positraction differential case and internals and ring gear drive pinion were subject to failure, and that's probably why your original axle is gone. Your car with L-76 and M-20 (close ratio only) could be ordered with a 3.08 Positraction axle and the code would be CJ. With M-20 a 3.70 open diff axle was standard with Positraction optional, and 3.08, 3.36, 3.55, 4.11, and 4.56 were optional, Positraction only.

L-76 could be ordered with the base three-speed manual. The only available axle ratio was 3.36, open being standard and Positraction optional.

You should download the 1963 Corvette Information package from the GM Heritage site. It has all the drivetrain configuration information and gobs of other useful data about the car. I also suggest you buy the NCRS C2 specification Guild.

Per the NCRS judging guidelines the date code must be within 6 month of the build date, which is very liberal for all but relatively rare options.

At about 30K miles the 3.08 ratio drive pinion of my 340 HP SWC snapped it two, and I had to have it rebuilt again at about 60K due to a spalled drive pinion bearing. I rebuilt it again in the mid-seventies at 115K and bought a new loaded Positraction case of the latest design from GM. The carrier and ring and pinion all passed Magaflux inspection so I reused them and assembled the axle to very tight tolerances including pinion bearing preload, three thou ring and pinion backlash, 3 and 5 thou side yoke end play and a good tooth contact pattern before I buttoned it up.

It's likely that somewhere along the line your OE axle was replaced with a used axle, and the '76 version is much stronger.

During Flight judging it's next to impossible to see the axle data with the car on the ground, but it can be inspected during Bow Tie judging.

Also consider that of all the various axle ratios, with and without Positraction. 3.08 Positraction made up about 10 percent of production. Now add to this finding a CJ carrier with a "proper" date code for your car and the odds of finding such are probably a little better than finding Jimmy Hoffas' body, but not much.

If you do pursue finding a '63 axle, you can choose any ratio since there is no way to know which ratio was originally installed unless you have original documentation. A 3.08 with the 340 or 360 HP engines and M-20 was not a popular choice since it's like a five speed with no first gear. It makes for a 150+ MPH sports car, but it's pretty doggy off the line and tough to get going from a dead stop with minimal clutch slippage. Have you verified that the '76 axle is still a 3.08. If the proper 18 tooth speedometer driven gear is installed, revs at 60 MPH are axle ratio times 760 = 2341 for a 3.08, regardless of installed tire size.

If you're looking to correct judging deductions, there are likely many that will be less costly and less hassle to fix.

Duke

[Henry S.]

Re: 1963 Corvette Correct Differential Designation.

Thank you Duke.
To be clear, the CB designation stamped into the differential case translates into a 3.36:1 ratio, with positraction, for a manual transmission or powerglide. The manual transmission referred to in this CB designation is a three speed, correct ? Separate designations CC CD CE CF CJ CX CZ, were used for four speed cars ?
Looks like we joined NCRS on the same day in 1991.

[Duke W.]

Re: 1963 Corvette Correct Differential Designation.

There was only ONE 3.36 Positraction axle in '63, code CB. If you look at the Powertrain chart you can see that it is optional on ALL engines with any available transmission, base 3-speed manual, optional 4-speed manual, and optional Powerglide. In fact, it was the most commonly installed axle in '63 accounting for nearly 32 percent of production.

Duke

[Bob R.]

[Duke W.]

Re: 1963 Corvette Correct Differential Designation.

A note on nomenclature. The complete axle is called axle. The large cast iron housing is the differential carrier, or simply carrier. The carrier "carries" the differential case, or simply case, thus the name differential carrier.

There are two case types. An open axle case contains the differential gears, and the ring gear bolts to the case. A Positraction case also contains the Positraction clutches and preload springs.

Assembling an axle begins with installing a case assembly complete with ring gear in the carrier that has the drive pinion installed, and once buttoned up you have a complete axle assembly to install on the vehicle.


Duke