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actmobmar 03-09-2013 05:58 AM

12 bolt gear change questions
just bought a 12 bolt /2.73 posi out of a '70 chevelle, i would like to change it to 3.73. took a look at jegs and see no mention of a 2 series carrier for fitment, i see 3 series but no 2 series, so are the carriers the same between 2 series gears and 3 series gears?

big gear head 03-09-2013 06:50 AM

No. The Chevy car 12 bolt uses a 2 series for 2.56 and 2.73 gears, a 3 series for 3.07 to 3.73 gears and a 4 series for 3.91 to 6.xx gears. There are special 3.73 gears that fit the 4 series and 3.91, 4.10, 4.56 and 4.88 gears that fit the 3 series. There are no 3.73 gears that fit the 2 series differential. You will have to get a 3 series Eaton Posi to use with the 3.73 gears, or you can find a 3 series Eaton Posi case and transfer all of the internal parts from your 2 series into it.

I know that someone is going to mention a ring gear spacer, so I might as well cover that too. Yes, you can use a spacer to put the 3.73 gears on your 2 series Eaton. Is it a good idea? NO! There are several problems that come up when using a spacer. The first one is that the ring gear no longer fits tight on the differential and will not center properly. This causes problems with the gear contact, backlash and other things. Another problem is that the ring gear bolts tend to shear off when using a spacer. I've seen these problems several times and I can not recommend using the spacer, even for a street car. Just do it right the first time and save yourself some money in the long run.

Kawabuggy 03-09-2013 01:26 PM

I agree with BGH on NOT using ring gear spacers.

In addition to the differential that BGH suggested, I could/would also recommend the Detroit Locker (Eaton manufactures these as well). They are helical type gear locking differentials. They can be had new for around $470 before tax & shipping and are brutally strong.

Make no mistake I am not bad-mouthing the Eaton clutch type posis as they were the cats meow back in the day, and are still a good choice today. They are so close in price though when you buy them new I would just go for the locker. I've installed both the clutch type & Detroit Locker in several customers cars and never a complaint or problem out of either.

big gear head 03-09-2013 07:16 PM

I think you have the Detroit Locker and the Truetrac mixed up. The Detroit Locker is a true locker that locks up using a dog type tooth ring and the Truetrac uses helical gears.

Kawabuggy 03-10-2013 04:08 PM

BGH you are correct! I just went and looked at the box from the last one we installed and it is a Truetrac!

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