GM 8.5 Rear Differential Master Overhaul Kit for Aftermarket Posi

Running an Eaton, Auburn, TrueTrac, Detroit Locker or other aftermarket carrier in a GM 8.5 inch 10-bolt and the rear has started to whine, growl or drip from the pinion seal? Or are you about to install one along with new gears? This USA Standard Gear ZK GM8.5-HD master overhaul kit is built for exactly that axle. It packages Koyo carrier bearings and races sized for aftermarket Positraction carriers with Koyo pinion bearings and races and every small part you need to set the axle up from scratch, so you are not caught with the wrong carrier bearings halfway through the job.

Why the HD version exists

The factory 8.5 open and GM posi carriers use one carrier bearing size. Many aftermarket limited-slips and lockers for this axle are machined with larger bearing journals, which is what the industry calls the heavy-duty or HD bearing pattern. Yukon's own technical note for its HD carrier kit lists the Dura Grip, Eaton Posi, ValueTrac, TrueTrac, Pro-Series Auburn and Detroit Locker, plus standard carriers with large journals, as the units that take these bearings. A standard 8.5 kit will not press onto those journals, so the HD kit is the one to buy when that kind of carrier is in the case or on the way.

What the master overhaul kit does in a GM 8.5

Everything in the center section rides on four tapered roller bearings. Two pinion bearings, held in preload by a collapsible crush sleeve and the pinion nut, locate the pinion gear fore and aft; the shim under the inner pinion bearing sets pinion depth. Two carrier bearings hold the carrier and ring gear, and the shims behind them set both side preload and ring gear backlash. If any of those bearings wears or the setup was never right, the tooth contact pattern drifts and the gears start to sing. This kit replaces the whole set at once. In the box: Koyo carrier bearings and races, inner and outer pinion bearings and races, pinion seal, a complete shim kit, ring gear bolts, pinion nut, crush sleeve, thread locker, marking compound with a brush, and the cover gasket.

Symptoms that call for this kit

• Whine that climbs with road speed, louder on acceleration or on coast
• A low rumble or growl from the center of the car that does not change in turns
• Clunk when you get on and off the throttle or shift into gear
• Pinion seal leaking gear oil onto the yoke and driveshaft
• Chatter or binding in tight turns from a clutch-type posi that is worn or on the wrong oil
• Fine metal paste on the drain plug or cover magnet

How to confirm before you buy

Drain the oil and pull the cover. Identify the carrier by the maker's casting or tag, then read the numbers on the carrier bearing cones or measure the journals. Large-journal aftermarket carrier: this kit. Factory carrier with standard journals: the regular GM 8.5 master overhaul kit is the correct one. While the cover is off, check the ring gear for chipped or pitted teeth and rock the yoke to feel for pinion bearing play. Damaged gears should be replaced during the same rebuild; a GM 10-bolt 8.5/8.6 3.73 ring and pinion is a common choice.

Fitment

This kit fits the GM 8.5 inch 10-bolt rear axle with an aftermarket Positraction carrier using the larger bearing journals. The 8.5 replaced the 8.2 in 1970 production and went under a long list of Chevrolet, GMC, Buick, Oldsmobile and Pontiac cars and light trucks, so confirm you have an 8.5 and not a 7.5, 8.2 or 8.6 before ordering; the cover shape, bolt pattern and ring gear size all differ. If you are unsure, send us a photo of the cover and your year and VIN. More axle and transmission kits are in our rebuild kits section.

Install notes

• Press the new carrier bearings fully onto the aftermarket carrier journals and the races into the caps and housing.
• Set pinion depth with the shims, then crush the new sleeve gradually until you reach the rotating preload in your service manual. Never back the nut off once the sleeve is over-crushed; start again with a new sleeve.
• Shim the carrier for backlash and preload, then paint the ring gear with the marking compound and read the pattern on both drive and coast sides.
• Use the supplied thread locker on the ring gear bolts and torque them to spec.
• Fill with the gear oil and any friction modifier the carrier maker calls for; clutch-type units are picky about this. A deep Yukon Hardcore 8.5 diff cover adds capacity and protection.
• Break the new gears in gently for the first few hundred miles and recheck for leaks.

Will this kit fit my stock GM 8.5 with the factory open or G80 carrier?

Usually not. This HD version is sized for aftermarket carriers with larger carrier bearing journals. A factory carrier with standard journals takes the regular GM 8.5 master overhaul kit. Check the numbers on your old carrier bearings to be sure.

Which aftermarket carriers use the HD bearings?

Yukon lists the Dura Grip, Eaton Posi, ValueTrac, TrueTrac, Pro-Series Auburn and Detroit Locker for the GM 8.5, along with standard carriers that have large bearing journals.

What bearings are in it?

Koyo carrier bearings and races and Koyo inner and outer pinion bearings and races.

Does it include axle seals or axle bearings?

No. It covers the center section: carrier and pinion bearings, pinion seal, shims, crush sleeve, pinion nut, ring gear bolts, thread locker, marking compound and the cover gasket. Axle bearings and seals are a separate kit.

Can I reuse my old ring and pinion with this kit?

Yes, if the teeth are not chipped, pitted or scored. Mark the old pattern before teardown and set the gears back up to match it. Worn or noisy gears should be replaced during the rebuild.

What gear oil should I use with an aftermarket posi?

Use the gear oil weight your axle calls for, usually 75W-90, and add friction modifier if the carrier maker requires it. Lockers and gear-type units often need none, while clutch-type units usually do.