Identifying the series of GM 12 bolt rear end...?

[Chrisw]

I've searched and tried to watch some youtube but am still trying to figure out how to identify what series of 12 bolt rear end I'm looking at. Found one for a decent deal somewhat local but... guy thinks is out of a GTO (which if I understand correct makes it a series 3) but also thinks it has 4.55 gearing (which would have to be a series 4 right)? How do guys actually know what series they are looking at.

I've got a 69 Beaumont with a 454 so I'm thinking I want 3.73 gearing (has to be a series 3 right?)...

So confused. Ha!

[chevymaher]

I am not sure if these are 10 or 12 bolts but this is how you tell. And your right finding this information is like a trade secret.

[Chrisw]

That is amazing. Thank you. Any chance you know what a 4 series would look like in comparison to these?

[chevymaher]

Nope I looked a long time for one. All we know is it isn't these 2.

[Chrisw]

Https://ibb.co/3R4bY8D

*wont let me imbed the photo for some reason*

So I finally got mine and pulled it apart. Gearing is a 4.56 so I'm assuming this makes it a 4 series. If my measurements are correct, its 1.82 from the flange to the end of the bearing.

It clearly has a large shim on the flange... to allow it to take a series three gear set I assume... although it brings it to about 2.02... which seems to be a hit heavy. My measurements might not be perfect though as I'm eyeballing that one.

[chevymaher]

Call richomnd gears tech dept. They were very helpful with my van.