GM 12-Bolt Truck Rear Master Bearing Kit, Timken Bearings

If the rear end under an older Chevrolet or GMC half-ton, a K5 Blazer or a Suburban is howling at cruise, or you are about to put a new ratio in a 12-bolt truck axle, this is the bearing kit that goes in with it. The Motive Gear R12RMKT is a master bearing kit for the GM 8.875 inch 12-bolt truck rear, built around Timken tapered bearings for the carrier and both pinion positions. It is the part of a gear job people try to skip and later regret: new gears riding on tired bearings will not hold a pattern for long.

What the bearings do in a 12-bolt truck rear

The 12-bolt truck axle locates the pinion with two tapered roller bearings, a large inner bearing behind the pinion head and a smaller outer bearing near the yoke, loaded against each other by a crush sleeve and the pinion nut. That preload is what holds pinion depth steady when torque tries to push the pinion away from the ring gear. The carrier rides on two more tapered bearings in the housing, and the shims behind them set both carrier preload and backlash. When any of these bearings wears, the gear mesh moves under load, the contact pattern wanders off the center of the tooth, and the gears start to sing.

The truck 12-bolt is not the same axle as the Chevelle, Camaro and Nova car 12-bolt even though both use an 8.875 inch ring gear. The truck pinion uses a larger inner pinion bearing (about 1.675 in. versus 1.438 in. on the car), the ring gear offset differs and the truck carrier is narrower. Car kits do not cross over, and that is the most common ordering mistake on this axle.

Symptoms of worn 12-bolt truck bearings

• A whine or growl that rises with road speed rather than engine speed, often loudest at a steady cruise
• A noise that changes or goes away between throttle-on and coasting, which points at pinion bearing preload
• A low rumble on long sweeping turns that sounds the same in both directions, more typical of carrier bearings
• In-and-out or up-and-down play at the pinion yoke with the driveshaft off
• Gear oil weeping at the pinion seal, often a follow-on of a loose pinion
• Fresh gears that went noisy quickly because the old bearings were reused

How to confirm before you buy

Put the truck on stands, pull the driveshaft and grab the yoke: any clunk or rock you can feel means the pinion bearings or the sleeve have let go. Spin the yoke slowly and feel for roughness. Pull the cover, drain the oil into a clean pan and look for glitter or flakes on the magnet. Our whine on acceleration versus deceleration guide walks through separating gear noise from bearing noise, and the pinion bearing noise guide covers what a failed inner bearing usually looks like once it is out.

Fitment

This kit fits the GM 12-bolt truck rear with the 8.875 inch ring gear, the axle used under 1963-1987 C10 and K10 pickups, G-vans, Blazers and Suburbans and their GMC versions. It does not fit the 12-bolt car rear, and it is a different axle from the later GM 9.5 and 9.76 12-bolt. Identify the housing before ordering: count 12 cover bolts, check for the truck-style cover and compare the axle code on the right tube. Our 10-bolt versus 12-bolt identification guide helps here. If you are not sure, send us your year and VIN and we will check it. Other axle bearing and install kits are in our rebuild kits section.

Install notes

• Motive Gear lists the R12RMKT master kit with Timken carrier and pinion bearings, cover gasket or RTV, crush sleeve or preload shim pack, marking compound with brush, pinion nut, pinion seal, ring gear bolts, pinion and carrier shims, and thread locker. Check the box against that list before you tear the axle down.
• Remove the C-clip axles, pull the carrier, then press the old bearings off and drive the new races in square. A bearing race and seal driver set keeps the races from cocking in the bore.
• Set pinion depth with shims, install the new crush sleeve and snug the pinion nut in small steps, checking rotating torque with an inch-pound wrench. The Yukon set-up chart lists 13-15 in-lb of pinion preload with new bearings for the 12-bolt truck, 55 ft-lb on the ring gear bolts and 60 ft-lb on the bearing caps. Do not back the nut off if you overshoot; start over with a fresh sleeve.
• Set backlash, then read the pattern with the marking compound on both drive and coast sides before sealing the cover.
• Seal the cover with the supplied gasket or RTV, or fit a fresh 12-bolt truck cover if the old one is bent or rusted through. Refill with 75W-90 gear oil, adding friction modifier if the carrier is a clutch-type limited slip.
• If the gears are worn too, pair this kit with a 12-bolt truck 3.73 ring and pinion, and follow the ring and pinion break-in procedure for the first few hundred miles.

Will the R12RMKT fit my Chevelle or Camaro 12-bolt?

No. This kit is for the 12-bolt truck rear. The car 12-bolt uses the same 8.875 inch ring gear size but a smaller inner pinion bearing and a different carrier, so truck and car bearing kits are not interchangeable.

Does this kit fit a GM 9.5 or 9.76 inch 12-bolt?

No. The 9.5 and 9.76 inch axles are later, different designs that also happen to use 12 cover bolts. This kit is only for the older 8.875 inch truck 12-bolt.

Do I need new bearings if I am only changing the gear ratio?

Yes, in practice. Setting up a new ring and pinion requires pressing the pinion bearing off the old pinion anyway, and putting new gears on used bearings is a common reason a fresh gear set gets noisy early.

What pinion preload should I set with new bearings?

Yukon's published set-up chart lists 13-15 in-lb of rotating torque for the GM 12-bolt truck with new pinion bearings. Check it with an inch-pound torque wrench as you tighten the pinion nut a little at a time.

What gear oil goes back in?

A 75W-90 GL-5 gear oil. If the axle has a clutch-type limited slip, add friction modifier unless the oil already contains it.