rear differential casting

[Rod W]

I own a 1976 L-48 Corvette with a TH-350 auto trans. It has VIN 1Z37L6S407784

I don't have the build sheet, but want to verify that the rear differential housing and gear ratio are "factory original".

How do I find this out?

Thanks!

Rod W Minnesota 4-19-00

[Joe L.]

Re: rear differential casting

Rod-----

To check if the differential housing is original, look at the bottom of the cast iron housing, just in front of where the rear cover attaches. There you should find a two alpha character code. Since 1976 Corvettes with L-48 and auto trans were only available with the 3.08:1 rear ratio, you should find code "OA" stamped into the case. Any other code and the carrier housing is not original to the car.

Even if the carrier housing DOES have the correct code, there is still the possibility that it is not original to the car. To further check, look for the casting date which is usually found on the left rear side of the housing. Usually, this date will precede the build date of the car by a month, or so, but may be as long as 5 or 6 months in some isolated cases. The sequence of the date code will be a letter followed by 2 or 3 numbers. The letter indicates the month of casting with A=January, etc. The next 1 or 2 numbers will indicate the day of the month. The last digit will be the last digit of the year or, for example, "6"= 1976.

Even if the carrier housing is original, someone could have changed the internal gears at some point in the past. To check for this, elevate the rear of the car and support on jackstands or other suitable devices. Then, mark the driveshaft with a crayon and mark the tire with a crayon. Have someone rotate the tire through exactly 1 complete revolution while you observe the driveshaft mark. The driveshaft should rotate just a little over 3 complete turns if the ratio is original.