Yukon Gear Master Overhaul Kit For GM 12 Bolt Truck Diff

A Chevy or GMC C10 pickup, G-series van or 1969-1981 Blazer with the 12-bolt truck rear has usually covered a lot of miles by the time it starts to howl. If yours whines at highway speed, clunks when you lift off the throttle, or leaves gear oil on the driveway under the pinion, the Yukon Gear & Axle YK GM12T master overhaul kit puts every bearing and seal in the center section back to new. It is built specifically for the truck version of the GM 8.875 in. 12-bolt, with Timken bearings and races.

What the master overhaul kit does in a GM 12-bolt truck rear

The truck 12-bolt shares its 8.875 in. ring gear size and its name with the famous Chevelle and Camaro rear, but almost nothing inside interchanges. The truck carrier is narrower, the ring gear offset is different, and the truck pinion uses a smaller inner pinion bearing, about 1.438 in. against the 1.675 in. bearing in the car unit. That is why the truck needs its own kit.

Inside the housing, the pinion rides on two tapered roller bearings held in preload by a collapsible crush sleeve and the pinion nut. The carrier spins on two side bearings, and shims behind them set backlash between the ring gear and pinion. Once any of those bearings wears, preload drops, the gear mesh shifts, and the contact pattern walks off the center of the teeth. The YK GM12T replaces carrier bearings and races, pinion bearings and races, the pinion seal, a complete shim kit, ring gear bolts, the pinion nut, crush sleeve, thread locker, gear marking compound with a brush, and the cover gasket. Gears and the carrier itself are not included.

Symptoms that call for this kit

• A whine or howl that rises with road speed, not engine rpm
• Noise that changes between pulling and coasting, which points to a shifting gear pattern
• A growl that gets louder with a load in the bed or a trailer behind the truck
• A clunk at the driveshaft when you shift between drive and reverse or get on and off the gas
• Wet gear oil on the yoke, driveshaft or frame crossmember from a leaking pinion seal
• Metal glitter on the drain plug or the inside of the cover

How to confirm before you order

First make sure it is a 12-bolt truck axle. Count the cover bolts, then look at the cover itself: the truck cover has an irregular shape, while the passenger-car 12-bolt cover is a smoother oval. If you have the car version, use the YK GM12P passenger-car overhaul kit instead.

With the truck raised and the driveshaft off, spin the pinion by hand. A gritty or notchy feel means worn pinion bearings, and any in-and-out play at the yoke means lost preload. Drain the oil into a clean pan, pull the cover and paint several ring gear teeth with marking compound to read the current pattern. Smooth, undamaged teeth mean this kit alone is the fix; chipped, pitted or blued teeth mean you need gears too.

Fitment

Yukon lists the YK GM12T for the GM 12-bolt truck differential. GM put the truck 12-bolt into C-series pickups and G-series vans starting in the mid-1960s, and it stayed available in light trucks until 1987; 1969-1981 Blazers used it in the rear. Over that span there were housing and spline variations, including early truck axles with coarse splines. Yukon also notes that early 1962-64 coarse-spline OEM gears use a shorter crush sleeve, part YSPCS-019 at 0.630 in. tall, rather than the one in this kit. Check the cover shape, ring gear size and your build year, or send us your year and VIN and we will confirm. More axle kits are in our rebuild kits collection.

Install notes

• Remove the cross-pin, push the axles in and pull the C-clips before the shafts and carrier come out
• Mark the side bearing caps left and right; they are line-bored with the housing and must go back in place
• Measure the factory pinion and carrier shim stacks before discarding anything and use them as the starting point
• Drive the new races fully home with a proper tool such as the 43-piece bearing race and seal driver set
• Bring the pinion nut up slowly against the new crush sleeve, checking turning torque as you go; overshoot it and the sleeve must be replaced
• Lock the new ring gear bolts with the included thread locker and check the pattern on both drive and coast sides
• Fill with 75W-90 gear oil, and add friction modifier if the truck has a limited-slip carrier

If the old gears are worn, set up a new ratio at the same time, such as the Yukon 12-bolt truck 3.73 gear set, and close it up with a fresh 12-bolt truck chrome cover if the old one is bent at the gasket face.

Will this kit fit a 1967 Chevelle or Camaro 12-bolt?

No. The passenger-car 12-bolt uses a larger inner pinion bearing, a different ring gear offset and a wider carrier. Car rears need the YK GM12P kit.

How do I tell a truck 12-bolt from a car 12-bolt?

Look at the cover. The truck 12-bolt cover has an irregular shape, while the car cover is a smoother oval. Truck carriers are also narrower and car axles will not fit a truck housing.

Does it include axle bearings or axle seals?

No. The kit covers the center section: carrier and pinion bearings and races, pinion seal, shims, ring gear bolts, pinion nut, crush sleeve, thread locker, marking compound and cover gasket.

My truck has early coarse-spline gears. Will the crush sleeve fit?

Yukon notes that early 1962-64 coarse-spline OEM gears use a shorter crush sleeve, part YSPCS-019 (0.630 in. tall). Check your pinion before assembly and source that sleeve if needed.

Can I reuse my original ring and pinion?

Yes, if the teeth show no chips, pitting or heat discoloration. Start with the original shims and confirm the pattern with the included marking compound.

What gear oil should I use after the rebuild?

75W-90 gear oil is the common fill for this axle. Add friction modifier for a limited-slip carrier, and change the oil after the first few hundred miles of break-in.