I have a 69 cutlass S. 12 bolt rear diff. When I change the fluid do use RTV sealant alone or use a gasket with the sealant?










heres several pictures of the gears. Will any of these numbers tell me the ratio and whether or not it’s a posi?

 

Looks like a 2.56 rear. "16 A 41" translates to: 41 ring gear teeth divided by 16 pinion gear teeth 41/16=2.56

Can't tell if it's an anti-spin carrier from the pics.
You can test by manually spinning the driveshaft and seeing if both tires rotate in the same direction

 

Before you put the cover back on, spin the diff and look into the spider gear area in the middle of the carrier. If you can see clutch plates on the outer edges of the carrier - toward the axles - and an "S" spring in the middle, it's Anti-Spin.

 

You can use RTV only, just make sure to thoroughly clean all the oil off the mating surfaces. When I use a gasket, I typically put a thin smear of sealant on both sides of the gasket, just to make sure it doesn't leak. I don't like leaking gear oil.

The Anti Spin (posi) carrier will have an S spring visible between the side gears as in the picture below. An open diff will not have the S spring.

https://classicoldsmobile.com/forums...-ratio-123000/

 

I have used just RTV on multiple diffs, including my 70S 12 bolt Type O.