can any one explain to me how to determine which series 12 bolt i have? i got the rear out of a 69 chevelle . i tubbed out my 70 nova,im ready to narrow this rear and i need to know in order to get the right gears for it. thanks.

To quote Freddie aka Big Gear Head: "The best way to tell what you have is to check the casting number on the posi itself. The number is next to the bearing on the side opposite of the ring gear. The 2 series is ED32088, the 3 series is EDB30140 and the 4 series is EDB30174. There are 3 carriers -2 series (2.73:1 & down), 3 series (3.08:1 to 3.73:1) and 4 series (3.90:1 & up)."

Big Dave

That takes care of that.

thanks guys that helped alot.