I want to figure out the ratio of the 10 bolt differential in this 65 Cutlass chassis. I've got the plate off and was going to count the teeth on the crown and pinion later. Not sure if I can easily see the pinion teeth, or maybe they're standard. Uncertainty prevails. I know the crown gear has this stamped on it- 4013 GM 565K A.
Do these digits tell us anything useful, or am I back to counting teeth? Any thoughts?

 

There is a code on the right axle tube that is two letters it is stamped upside down between the center section and the right suspension mounts. That will tell you what it was when new. I can decode that for you. Another way is count the teeth on the ring gear and count the teeth on pinion and divide them. That wil give you the ratio. Also you can rotate an axle two turns and count the rotation of the pinion yoke. That will also work.

It looks like yours is 3:08 based on the numbers you provided and below is a breakdown of that. You should have a code of SC or SD depending on whether is it a open or posi carrier. Keep in mind that is all if it is stock.

Hope this helps.

Larry

This might help also.
Ratio teeth open locking
2.78 39-14 SA SB
3.08 40-13 SC SD
3.23 42-13 SE SF
3.55 39-11 SI SJ

 

Excellent info Larry, thanks. Should be under the car again in a few hours to continue the adventure. It's an open rear. I used to just turf out my non-posi units unless they were heavy duty, but the stuff is getting older now, and I'm inclined to save more.

 

hey dave, mine has a # with a decimal point you can decode, its around the outside of the ring somewhere i think, maybe thats the 4013# you have, 40 big teeth and 14 on the pinion gear?

IMG_0213.jpg

 

I think you're right, Stan. The numbers were on one part of the crown or ring gear. Looks like it's a 3.08 but just for fun I'll check the code on the right tube in a bit. Being retired gives me time to play with all this important stuff.

 

That 4013 should mean 40 ring gear teeth and 13 pinion gear teeth.

40 / 13 = 3.0769 or 3.08 as we know it.

 

Quite right. I counted the teeth. Worked out exactly to 40 on one gear and 13 on the other. Thanks for the help, guys.