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Rebuilding a 12-Bolt Rearend to Hold More Power

TODAY’S BUILDERS demand a lot for their trucks, and this goes especially for the power segment. Getting performance has been more attainable in the last few years and has increased our expectations. There is a huge selection of crate engines on the market, and stout salvaged versions are plentiful.

With great power comes the question of sticking it to the ground, since your truck is only as good as its weakest link. For most that point is the rearend. Fortunately, C-10s have decent versions that can be modified to suit modern demands.

Chances are your truck has a 12-bolt rearend, but if not, it’s fairly easy to get your hands on one. Not only are they common, but they can also be pretty solid when built correctly. In fact, they can stand up to around 800 hp, which is perfect for a street truck with some performance mods under the hood. Thirty-spline axles and a large ring gear help these rearends hold power. They also require less power to turn because of the height where the pinion gear contacts the ring gear.

Since they’re plentiful, a 12-bolt can be found for only a few hundred bucks, but it’s still important to make sure you’re getting one that’s in good condition. While you’re modifying them to fit your needs, you might as well change some of the internals while you’re at it, including the bearings, seals and even the gears.

We had a 12-bolt and decided to do a full rebuild. Not only did we want to make it solid, we also wanted to change a few things. We searched Summit Racing’s website to find all of the Yukon Gear and Axle components we needed. We started with 31.5-inch axles (P/N YA G14071751) to convert from a 6- to 5×5- lug pattern. To help our truck jump off the line better, we changed the gears to 4.11:1 (P/N YGA-24130). To make the truck run like new, we ordered a master rebuild kit (P/N YK GM12T).

One drawback to running a factory rearend is the open differential. It will allow the wheels to rotate at different speeds for a smooth ride, but it will also transfer power to the wheel that has the least traction. This isn’t ideal when you’re trying to get the maximum amount of power transferred to both rear wheels. To combat this, we ordered a Yukon Dura Grip positraction differential (P/N YDGGM12T-4-30-1), which uses a set of springs to lock two packs of clutches in order to distribute power more evenly between the two separate wheels.

With our rearend set up to help the truck go faster, we thought it would be wise to help it stop shorter too. When it comes to getting better stopping power, Wilwood is at the top of the game. We ordered an Aero6 disc brake kit (P/N 140-15305) for the front with six-piston calipers and 14-inch two-piece rotors. For the rear, we matched it with an Aero4 rear disc/ parking brake kit (P/N 140-10941) with four-piston calipers and 14-inch two-piece rotors.

The included SRP rotors have a drilled and slotted pattern to improve braking response and pad performance. The holes and slots vent and clean to reduce pad glaze and disperse gases and heat. The rear rotor hats also have an internal shoe system that can be used for parking.

To help us build our rearend, we took everything to J and S Gear in Huntington Beach, California. John Coulman is the man with a plan with more than 30 years of experience building rearends. Not only is he a top-notch rebuilder, he can also narrow rearends to fit specific wheel/tire combos. While talking with John, we discovered that he got into his profession by doing what most of us do: hammering on our vehicles and breaking parts. After watching him build this 12-bolt rearend in just a few hours, we can confirm that he knows his stuff. After a simple rebuild, our rearend was ready to go, moving our project faster and stopping it better.


The original rearend was completely disassembled, and the sludge was removed in a hot tank at J and S Gear. Then it was sent to get media-blasted to expose the bare housing. Later it was coated in primer and black paint.

Grease was applied to the rearend before the pinion races were installed using a race driver; this tool can be purchased from Summit Racing.

The carrier bearings were pressed on with a hydraulic press. John recommends using a press to prevent damage.

The ring gear was attached to the carrier, and the bolts were secured to 55 ft-lbs of torque.


John used a .030 shim and slip-on bearing for set-up purposes.

The outer pinion bearing was installed on the rearend before the pinion was dropped in place.

To keep it in place, the yoke was given a gentle tap to snug it on the splines.

Races were put on the carrier bearings and the unit was dropped in. The backlash was set with a carrier .235-inch shim on the driver’s side and a .245-inch shim on the passenger’s side.

The caps were snugged on to check the setup.

A dial indicator was used to check backlash and contact pattern between the ring-and-pinion gear. The dial read .007 inch of backlash, which is within the .006- to .010-inch specifications. White lithium grease was applied as well to check the contact patch. As you can see, the pattern left behind was nice and even.


Since the setup was within spec, the slip-on bearing was removed, and the actual bearing was pressed on to the pinion. Then the crush sleeve was put in place before the pinion was installed.

The pinion seal was installed before the yoke. Afterwards, the pinion nut was torqued to 13-15 in-lbs of rotational torque (force required to move the pinion).

The caps were put in place, and then the bolts were torqued to 60 ft-lbs.

Gear-marking compound was used to check the contact patch and backlash again to ensure it stayed in place during final assembly.

A set of axle bearings was placed in the rearend using the proper installation tool.

The bearings were greased, and a set of seals was installed.

Wilwood backing plates were bolted to the flanges on the rearend housing.

At this point, the pinion-retaining bolt was removed, and the carrier-pinion shaft was pulled so that the new axles could be installed later.

The wheel studs (P/N YGA-33188) were set on the axles with the press.

The axles were slid in the housing and lithium grease was applied.

C-clips (P/N YSPCC-010) were installed to keep the axles in place.

The carrier-pinion shaft was put back in and the retaining bolt was installed.

RTV liquid gasket was used to fill divots developed through aging. Then the gasket and another layer of RTV were applied to create a good seal.

A Spectre Performance aluminum diff cover (P/N 60769) was attached with the provided bolts and torqued to 15-20 ft-lbs.


The two pieces of the rotors were bolted together. Safety wire was used to prevent the bolts from vibrating loose.

Once the rotors were installed, the caliper brackets were attached, and the hardware was torqued to 20-ft-lbs.

Brake pads and four-piston calipers were secured in place.

John welded in tabs to secure the stainless brake hoses.


Finishing it off , 4.5 pints of Lucas 85w 140 mineral-based oil were added along with GM limited-slip additive. Once complete, we had a rearend that was strong enough to hold decent power and transfer it to the rear wheels for maximum traction. Thanks to the 14-inch four-piston brakes, we have ensured that our C-10 will have no problems coming to a halt.