the numbers you've provided are casting numbers, we need the stamping numbers. They're stamped on the tube that goes from the differential center section to the passenger side backing plate, about 2" (5 cm) from the center section. The stamping faces forward, and you'll likely need to take a wire brush to clean the rust/grime/etc away. They'd be on the left side of the photo that shows the GM2 casting.
See what you can find, and once youy've got it cleaned away, it helps to rub some chalk into the numbers then wipe it off. Tell us the year of the diff, and we should be able to tell you the ratio and whether it's posi or open.
Rear Axle Code information is stamped on the axle housing for this year.. contains a 7 digit code that tells the stock ratio, month / day of build as well as the production facility, either W - Warren or B - Buffalo Format is: CA0403B for a 3.08 rear ratio built on April 3rd at the Buffalo plant.
Thanks 67Poncho for letting Micky know where to look. "CP" in the CP 1214K is the meaningful code, and they will let you know the ratio and whether it's open or posi. 1214 is likely Dec 14. What year model car is the diff from??? I can look it up from there. - now that I think about it, my books are packed up as part of a basement reno, but I can get 67rag396 to look it up.
'64-'72 10 bolts were 8.2 ring gear '73 on up are 8.5 ring gears
Dave; Micky's does look like a 8.2 with that cover.. My Ventura has a 8.5 with the tabs pointing down on the cover... I will have to look more, as I thought the 8.5 was more common ... Micky, the name McKinnon was the place where the rear was built in Oshawa....
