I need to know a little info about the 12 bolt car rearends. How many different carriers were there in them, and does anyone know what the gear ration range was for each carrier without using a spacer plate.

Thanks
Keith

Keith, Big Gear knows that answer, and he really helped me out when trying to upgrade to another gear, but as an aside, I think you're on the right track when you say, "without a spacer plate". I have not heard anything positive in using one.

The 12 bolt has 3 different series of differential cases. The 2 series is for 2.56 and 2.73 gears, the 3 series is for 3.07 to 3.73 gears and the 4 series is for 3.90 to 6.14 gears. There are exceptions to this rule. You can get a 3.73 set that has a thin ring gear to fit the 4 series case. You can also get 3.90, 4.10, 4.56 and 4.88 sets with thick ring gears to fit the 3 series differental. All of these fit into the same rear end housing.

Thanks Jethro and big gear head. Now, there is a difference between the 12 bolt car and truck rearends, right? Something about the pinion bearing if I remember right.

The pinion shaft is smaller in the truck rear end. The ring gear flange also has a different offset on the differential.

ok. that's what I was wondering. Thanks again for the answers. I think I know what I'm looking for now. I'm thinking of either a 3.73 with a series 3 carrier if I'm going to run 255/60/15's... or a 4.10 with a series 4 carrier if I decide to go with 275/60/15's... Now I just have to decide on what tires..

Keith,
I have a 3 series Eaton with a 3.90 gear. I run 255/60/15's on 15x8 vette rally wheels. But at the track I am switching to 275/60/15 BFG Drag Radials on another pair of 15x8 rallies.
Much better than the 3.08 worn out posi that I had.
Freddie helped alot in getting the parts and advice on the set-up of the gears. Everything works as advertised. graemlins/beers.gif

I would recommend using the 3 series posi with a thick 4.10 gear instead of the 4 series posi. It gives you much more gear ratio options in the future if you decide to do more street driving.

Yea, it sounds like the 3 series is the way to go after seeing what is available out there. I wish I could say I was looking for this for an old camaro, but I recently traded for a 70 Cutlass Supreme with a 12 bolt 2.56 gear. (I'm Chevy through and through, but for the condition of the car, I couldn't pass it up.)