Differential Casting Numbers

**lhdg865** · 01-27-13, 08:11 PM

Can anyone help me decode the casting numbers on my 68 Coupe differential? The numbers cast on it are as follows: 2 AT 71 W. Thanks for any help.

**6880 Mike** · 01-28-13, 08:30 AM

AT is the code for 3.08:1 posi for big blocks; W indicates the Warren, Michigan plant. I believe the 2 is for the second shift and 71 is the year.

**lhdg865** · 01-28-13, 05:00 PM

Originally Posted by 6880 Mike

AT is the code for 3.08:1 posi for big blocks; W indicates the Warren, Michigan plant. I believe the 2 is for the second shift and 71 is the year.

That is what I was thinking Mike, but I did not see the AT as an option code in 1971. I know it now has 3.70 gears but was just curious as to what it may have had in it at birth. Thanks for the information.

**Vettehead Mikey** · 01-28-13, 05:51 PM

I believe the '71' would be the 71st day of the year the diff was assembled. What's the casting date of the case itself?

**lhdg865** · 01-28-13, 07:37 PM

Originally Posted by Vettehead Mikey

I believe the '71' would be the 71st day of the year the diff was assembled. What's the casting date of the case itself?

I do not have that number. Can it be viewed from under the car or does the differential have to be out? Thanks.

**GTR1999** · 01-29-13, 06:30 PM

The casting # is on the lower LH side of the diff. A letter and 2-3 numbers- A 238 = Jan 23 1968 or 1978.

If you have a 71 chances are someone blew up the original 68 posi case. The 63-64 & 65-69 cases were not the best. The 3's 4's were junk, the 5-9" not much better. The '71-9 cases were better, had better spiders, but lousy clutches. Sounds like someone was in there, just hope they built it correctly and addressed the weak areas in there.

**lhdg865** · 01-29-13, 09:23 PM

Originally Posted by GTR1999

The casting # is on the lower LH side of the diff. A letter and 2-3 numbers- A 238 = Jan 23 1968 or 1978.

If you have a 71 chances are someone blew up the original 68 posi case. The 63-64 & 65-69 cases were not the best. The 3's 4's were junk, the 5-9" not much better. The '71-9 cases were better, had better spiders, but lousy clutches. Sounds like someone was in there, just hope they built it correctly and addressed the weak areas in there.

Thanks to everyone for the good information. I'm going to crawl under this weekend and check out the other numbers. I guess it really doesn't matter one way or the other, but I am always interested in finding out a bit more history on my car. Thanks again.

**GTR1999** · 01-30-13, 11:38 AM

Originally Posted by lhdg865

Thanks to everyone for the good information. I'm going to crawl under this weekend and check out the other numbers. I guess it really doesn't matter one way or the other, but I am always interested in finding out a bit more history on my car. Thanks again.

Here it is, this is a cleaned up diff I built for a guy- Now for you sharpies out there what year is this for?

**lhdg865** · 01-30-13, 05:20 PM

Originally Posted by GTR1999

Here it is, this is a cleaned up diff I built for a guy- Now for you sharpies out there what year is this for?

Let me see if I have learned anything...How bout October 18th 1963 or 1973???

**GTR1999** · 01-30-13, 08:08 PM

Originally Posted by lhdg865

Let me see if I have learned anything...How bout October 18th 1963 or 1973???

That is correct it is a 10/18/63 casting used in a 1964 corvette. The thing that really confirms it as a 63 housing, by just that picture, is the front bracket hole is tapped 7/16-14 where as 65-79 housings had thru holes for both bolts.

Good job