1963 differential

[Alan Weeks]

I am looking to have my differential rebuilt, I want to change it from a 4:11 to a 3:36. I have been told that the differntial could be exchanged for a 1965 due to unavailability of 1963 parts. Will this cause any problems, is there anything I should be aware of before I ship this unit out to Virginia vettes?
Thanks for any input
Alan

[Joe L.]

Re: 1963 differential

Alan-----

You have been told correctly. Some 1963-64 internal parts for the differential are different than 65+. Parts for the posi-traction unit are among those which are different and the 63-64 parts are generally unavailable. In your case, if you want to change from a 4.11:1 to a 3.36:1, then you need to change the posi-traction unit, anyway (or "convert" the positraction unit). In any event, changing the complete positraction unit is, by far, the most practical thing for you do do.

The complete GM unit is discontinued, but you can obtain a brand new unit from Eaton (the original manufacturer of the 65+ units). Also, you could obtain a used, rebuilt Eaton unit from many sources. Of course, you want the "3-series" positraction unit which is used for differential ratios in the 3.XXX:1 range. The unit that you have currently is a "4-series" and, if it's original to the car, it will be a Borg-Warner type unit (which is the one for which service parts are difficult/impossible/expensive to obtain.

You will also obviously need a ring and pinion gear-set since you want to change ratios. This is another case in which the 1963 parts are, to my knowledge, no longer available. 1963 used a different pinion gear spline configuration. So, you can obtain a 65-79 gearset which is easy to obtain and completely retrofittable to your case.

Last, you will need to change the pinion (companion) flange. This is the flange which mounts to the front of the pinion gear shaft and to which the driveshaft rear u-joint attaches. Since the pinion gear spline count is different, the pinion flange is different. The one you need is GM #3878972 and is still available from GM. It will work perfectly and is otherwise configured very similar to your original.