Better Axles for GM 12 Bolts

To complete our 12-bolt conversion, we also added a Detroit Truetracgear-style limited-slip unit from Tractech. The gear-style limited slipoffers several advantages over clutch-type units, including moreconsistent performance over a much longer period of use and abuse.

Since this move required a complete disassembly of the rear gears, wealso decided to upgrade our abused Eaton clutch-type Posi with a newTractech Detroit Truetrac gear-style limited slip. The gear-stylelimited slip is designed to apply maximum torque to the wheel that canhandle the additional power without spinning. This will allow us toapply much more power coming off the corner on an autocross or roadcourse without spinning the unloaded tire.

We also decided to improve our 12-bolt's durability with a Ratech solidpinion spacer that replaces the crush sleeve. This solid spacermaintains a more accurate preload on the two opposing pinion bearings,which makes life easier on the pinion gear. We also used a completeRatech ring-and-pinion installation kit.

We also decided to upgrade our 12-bolt with a couple of other extras,including an installation kit from Ratech that includes new bearings,seals, and shims to set up both the pinion depth and the backlash. Wealso use a Ratech solid pinion spacer instead of a crush sleeve toestablish a more consistent pinion position relative to the ring gear.This requires a little more setup time, but the increased durability iswell worth the additional effort.

This conversion will radically improve the stock 12-bolt's strength andaxle retention when combined with a stronger Moser aftermarket axle. Nowthat Moser has pumped a bunch of strength into our 12-bolt, it's time tohit the track and start abusing our Chevelle all over again.

We actually converted two different 12-bolt housings to the Moser designand used Ratech rebuild kits for both that included pinion and carrierbearings plus other necessary components to complete the reassembly. Wedecided to retain the original clutch-style Posi in the second rearend,so we also upgraded the internals with Ratech pieces. We've discoveredthat the cross pin will wear against the side gears, causing clearancethat reduces the preload on the clutches. Replacing the spider gears andcenter pin is a big step toward optimizing an Eaton Posi.

Our pal Tim Moore used his T&D Machine ring-and-pinion set-up tool toaccurately dial in our 12-bolt with the proper pinion depth and backlashfor the 3.42 gear.

Once Tim had installed the ring-and-pinion centersection, we installed aset of Baer rear caliper mounts for the disc brakes, slid the Moseraxles in place, buttoned it all up, and were ready to go. Moserrecommends breaking the new gears in with 50 to 100 miles of easydriving before subjecting the rear to performance use on the dragstripor the road course.