USA Standard Master Overhaul Kit For The GM 12P Diff
A 12-bolt car rear that howls on the highway, clunks when you get on and off the throttle, or drips gear oil from the pinion yoke needs new bearings and a fresh setup, not just an oil change. The USA Standard Gear ZK GM12P master overhaul kit gives you every bearing, race, shim and seal for the GM 12-bolt passenger-car differential in one box, at a lower price than the Timken-bearing version. It is aimed at the Chevelle, Camaro, Nova, El Camino and Impala crowd rebuilding a rear that has been in service for decades.
In the GM 12-bolt, the pinion gear rides on two tapered roller bearings held in preload by a crush sleeve and pinion nut, and the carrier spins on two side bearings whose shims locate the ring gear against the pinion. Gear noise comes from that geometry drifting: bearings wear, preload drops, the pinion walks, and the contact pattern moves off the center of the teeth.
USA Standard lists the ZK GM12P with Koyo carrier bearings and races, Koyo pinion bearings and races, a pinion seal, a complete shim kit, ring gear bolts, a pinion nut, a crush sleeve, thread locker, marking compound with a brush, and the cover gasket. With all of it in hand you can set pinion depth, preload and backlash from scratch instead of reusing parts that have already worn in. Ring and pinion gears and the carrier itself are not included.
Make sure it really is a car 12-bolt: count twelve cover bolts and look for the oval car cover rather than the irregular truck cover, which takes the 12-bolt truck overhaul kit. Then raise the rear, drop the driveshaft and rotate the pinion by hand to feel for roughness, and rock the yoke for end play. Pull the cover, paint the ring gear with marking compound and read the pattern. Healthy teeth with bearing noise mean this kit is the fix. Chipped or pitted teeth mean you also need a gear set such as the 12-bolt car 3.90 ring and pinion.
The kit is listed for the GM 12-bolt passenger-car differential with the 8.875 in. ring gear, the rear used under GM performance and full-size cars in the 1964-1972 era. The car and truck 12-bolts share a name but not bearings or carriers, so do not assume a rear is original to the car it is under. Check the cover shape and housing casting, or send us your year, model and VIN. If you would rather have Timken bearings, see the Yukon YK GM12P kit. More differential parts are in our rebuild kits collection.
| Brand | Yukon Gear & Axle |
| Part number | ZK GM12P |
| Fits | USA Standard Master Overhaul Kit For The GM 12P Diff |
| Type | Differential Overhaul Kits |
| Condition | New |
| Ships | Same business day (in stock), from the USA |
| Warranty | 1-year |
Both are master overhaul kits for the GM 12-bolt car rear with the same type of contents. The ZK GM12P from USA Standard Gear uses Koyo bearings and races, while the Yukon YK GM12P uses Timken bearings and costs more.
No. It is for the passenger-car 12-bolt. The truck 12-bolt uses different bearings and a different carrier and needs the truck kit.
No. The kit has the bearings, races, shims, seal, crush sleeve, ring gear bolts, pinion nut, thread locker, marking compound and gasket. Gears and the carrier are bought separately.
The kit includes new ring gear bolts, so there is no reason to. Install them with the supplied thread locker and torque them to spec.
Use 75W-90 gear oil. If the rear has a Positraction or other clutch-type limited slip, add friction modifier unless the oil already contains it.
It can be if you have a dial indicator, an inch-pound torque wrench, bearing pullers and a press. Setting pinion depth, preload and backlash correctly is what makes it quiet, so take your time on the setup.
