Axle codes / casting

[Jake1074]

Are these axle casting numbers of any help identifying the rear differential?

Passenger side next to fill plug - 3896838
Drivers side - GM 66

I tried to visualize the top of the right side for stampings but didn't see any may be too dirty.

It's in a '67 that has been converted to 5 lug - open rear. The SPID was a 3.73 posi rear, but apparently no longer.

[Mbeef61]

What are you trying to find gear ratio? Open diff or posi?

I just had to open mine and look. You can count teeth or find the numbers on outside of ring gear and divide. Mine are 3.73 (41/11).

There are ways to spin wheels and count drive shaft revolutions and wheel revolutions etc, but for me I wanted to see the condition of everything in there too. To me codes were no good because things had been changed

[Jake1074]

Originally Posted by Mbeef61

What are you trying to find gear ratio? Open diff or posi?

I just had to open mine and look. You can count teeth or find the numbers on outside of ring gear and divide. Mine are 3.73 (41/11).

There are ways to spin wheels and count drive shaft revolutions and wheel revolutions etc, but for me I wanted to see the condition of everything in there too. To me codes were no good because things had been changed

Attempting to determine the ratio. Another site listed the casting # as an open rear - 3.07 ratio from a 71 C 10.

[Mbeef61]

Easiest way for me was to open it up and look and check the condition. You can look up ways to determine ratio by jacking up the rear. In neutral. Mark the drive shaft and spin wheels forward one rotation and count the number of times the drive shaft rotates. If it’s 3.07 it should be just over 3 driveshaft rotations.

Many times the casting numbers are inaccurate because someone changed something in the last 40-50 years