Rear End Pedigree

[Mike B.]

Yesterday I removed the drop out differential from my '61 Corvette as I was finally able to find a date and casting code correct carrier. My plan is to swap out the posi unit, replace the bearings, and change the incorrect 3.55 gear set to a correct 3.70 set.

When I pulled it apart I was surprised to find an original GM ring and pinion both dated June 1957. All pieces rode in Hyatt bearings which I know was a GM company (my father-in-law spent his career there). The posi is a correct Armor Steel 3 series although I could find no date on it but everything seems to have been created an installed at the same time.

It was all mounted in a casting number 3725899 carrier dated F57 (which jibes with the gear set dates). BUT the carrier is not a P case. Does anyone know if early on GM did not mark posi cases with the P designation? Or could it be that the gear set and posi unit was swapped into a different case at some time, and that the date alignment is just a coincidence?

Also, from what car(s) was that 3725899 carrier originally installed?

Thanks in advance for your help.
Mike Bovino

[Terry D.]

Re: Rear End Pedigree

Mike
The 899 started in 1957 and went thru 1962 in the Corvette and thru 1964 in the passenger car. It was always an open rear end. 57 thru late 61 posi was an 833, replaced in late 61 by the 812. The "P" stated after April 1st, 1957. Info from Corvette by the numbers.
Hope this helps
Terry

[Mike B.]

Re: Rear End Pedigree

Thanks Terry. From your info it appears that the posi unit was swapped into the non-posi carrier.

I recall that a section of internal webbing on a non-posi carrier has to be removed to accomodate the posi. I will look for any evidence of that change. I love sticking my hands and nose in that gear oil!

I have no idea how many miles are on the assembly but the wear pattern is perfect and the bearings are in great shape. Nothing like USA made 50 years ago.

Mike