12 Bolt carrier size?

[70GMCer]

I have an open 12 bolt on my 70 w/373 gears. Plan on installing a posi and was wondering if the 373 gearing will except a 2 or 3 series carrier or either one?

[powell72]

You can get 3.73's in a 3 or 4 series carrier. Our trucks didn't have any 2 series gearsets.

[special-K]

How about 3.42s?Which carrier is required for that ratio?

[70GMCer]

So either a 3 or 4 series will work with that gearing?

[BowTieOgre]

3.73 and up is a 4 series but I think I heard you could put 3.73 gears in a 3 series as long as you added some sort of spacer. Unless you ever plan on going with a 3.08 or something later on just get a 4 series and call it good.

[71chevylowrider]

if I remember correctly, a series 3 does 3,07/08 to 3.73 & a series 4 starts at 3.73 & goes up to 4.56

[mnunn454]

Originally Posted by 71chevylowrider

if I remember correctly, a series 3 does 3,07/08 to 3.73 & a series 4 starts at 3.73 & goes up to 4.56

With Chevy car rear ends the breaking point is different (numerially lower), but for our trucks, 3.42 and down take a 3 series carrier. 3.73 and up take a 4 series carrier, just as BowTieOgre said.

When I bought my Eaton posi and new 3.73 R&P I had to make the tough choice. I opted for performance over mileage and went with the 3.73 over the 3.42, and therefore had to go with the 4-series carrier. Since upgrading to the 454, now I wish I'd gone with the 3.42s and 3 series carrier.

You can run a 4 series R&P on a 3 series carrier, if you use a ring gear spacer or buy a "thick" style ring gear. This arrangement should be fine for stock/normal use.

Check with the R&P sellers if you're still in doubt.