What year was this rear end made? ?Any Idea's?

[Larry E.]

Pictured is a FC rear end which we all know is a 4:56 ratio. Date looks to be in "Julian" characters which indicates it was
produced in February. AFAIK Julian date codes started in 1971 but a 4:56 ratio code was AD not FC. So it boils down to
this: Could it have been made say in 66 or 67 and being so rare it was coded with julian characters or if it was a service
rear end and made say in 1971 they used a older code of FC then AD? Thanks in advance--Larry

[Jeff P.]

Re: What year was this rear end made? ?Any Idea's?

Larry i see this FC code only up to 1969. I guess maybe a 66-69 only.

[Larry E.]

Re: What year was this rear end made? ?Any Idea's?

Follow Up question for Joe Lucia or anybody that might know:

On a service part when made(Let say 1971) would they use the current code.(In this case AD) or would they use
the previous code. (FC) Thanks again: Larry

[Wayne M.]

Re: What year was this rear end made? ?Any Idea's?

Another clue would be the casting date of the diff case (found on driver side, just above the edge where the spring is bolted, and below and a little forward of the LH output yoke).

In this case (no pun) the "I_11_8" is Sept 11th 1968.

[Joe L.]

Re: What year was this rear end made? ?Any Idea's?

Follow Up question for Joe Lucia or anybody that might know:

On a service part when made(Let say 1971) would they use the current code.(In this case AD) or would they use
the previous code. (FC) Thanks again: Larry

Larry-------

The differential code assigned to Corvettes defined the configuration of the complete assembly This included pinion flange, stub axles, and cover. I don't know if the SERVICE assemblies included all these components (I never purchased a complete differential assembly although I did purchase all of the NOS components to assemble one). If they did include all of the parts included as part of the PRODUCTION assembly, then I would expect the code to be the same. If they did not, then I would expect the code to be different or no stamped code, at all.

If the SERVICE assemblies included the pinion flange (and I believe they did), then the only 68-70 assemblies that would have been the same as 71-79 would be those used for THM-400 applications. No 63-67 could have been SERVICED with a 68-70 THM differential assembly or any 71-79 differential assembly.

As far as I can tell, however, none of the PRODUCTION differential assemblies were ever available in SERVICE. The SERVICE part numbers were different and this implies there was a difference in the parts content of the PRODUCTION versus SERVICE assemblies and, therefore, a difference in codes if any codes, at all, were used on the SERVICE assemblies. In addition, I cannot find that any 1971-79 Corvette differential assembly was EVER available in SERVICE.

[Larry E.]

Re: What year was this rear end made? ?Any Idea's?

Larry-------

The differential code assigned to Corvettes defined the configuration of the complete assembly This included pinion flange, stub axles, and cover. I don't know if the SERVICE assemblies included all these components (I never purchased a complete differential assembly although I did purchase all of the NOS components to assemble one). If they did include all of the parts included as part of the PRODUCTION assembly, then I would expect the code to be the same. If they did not, then I would expect the code to be different or no stamped code, at all.

If the SERVICE assemblies included the pinion flange (and I believe they did), then the only 68-70 assemblies that would have been the same as 71-79 would be those used for THM-400 applications. No 63-67 could have been SERVICED with a 68-70 THM differential assembly or any 71-79 differential assembly.

As far as I can tell, however, none of the PRODUCTION differential assemblies were ever available in SERVICE. The SERVICE part numbers were different and this implies there was a difference in the parts content of the PRODUCTION versus SERVICE assemblies and, therefore, a difference in codes if any codes, at all, were used on the SERVICE assemblies. In addition, I cannot find that any 1971-79 Corvette differential assembly was EVER available in SERVICE.

Joe: If I read you correct with your explanation then it sounds like this rear end was made when the FC code was valid. But then the question remains why
the Julian code instead of the monthy codes when the FC code exited? Thanks Again--Larry

[Joe L.]

Re: What year was this rear end made? ?Any Idea's?

Joe: If I read you correct with your explanation then it sounds like this rear end was made when the FC code was valid. But then the question remains why
the Julian code instead of the monthy codes when the FC code exited? Thanks Again--Larry

Larry------


The 1966-70 Corvette with 4.56:1 gear ratio differential is one of the assemblies that was available in SERVICE. The part number was GM #3899141. Whether this unit carried the "FC" stamped code, I do not know. If it was the identical assembly that was used in PRODUCTION, then it probably did carry that code. GM #3899341 was discontinued without supercession in 1977.

[Larry E.]

Re: What year was this rear end made? ?Any Idea's?

OK; thanks to everybody that responded. This will be the last time I push this to the top of the list.
Does anybody have a picture of a FC Rear End Code that can be posted? Still trying to find out the bottom
line of "Julian Date" vs "Monthy Dates" in the 1966-1970 period. Thanks Again--Larry