Gold, whether struck by luck or skill, is still gold. And whether GM intended the 14-bolt to be the ultimate in affordable, unbreakable axles or it was simply a fluke we’ll never know. Either way, we’re sure glad. The axle came from the factory in more vehicles than we can count. For the average Joe, it’s more than up to the task of big tires, bigger rocks, and even a good bit of power and torque. The only downside is the hefty weight and the limited ground clearance afforded by its massive 10 1/2-inch ring gear—the latter being both a blessing and a curse.
One way to trim some of the 14-bolt’s pork is a quick and easy disc brake swap. The standard-issue drums are the size of Bigfoot’s food bowl and weigh a ton! It’s a no-brainer to ditch them in favor of lighter, better-stopping, and easily serviceable disc brakes. Another homerun by the GM crowd is the back-and-forth parts compatibility that allows easily sourced front rotors and calipers to be repurposed at the rear. The job can be tackled in the driveway in a few hours. Better yet, it’s a completely bolt-on affair.
Our 14-bolt specimen came from a 1985 M1028 diesel military truck and has seen its share of miles, abuse, and geographically classified road grime. On the bright side, it came with 4.56 gears and a Detroit Locker. Now being rebuilt for a second life as an off-roader, the first step is to ditch the drums. Follow along as we detail the process of retro-fitting disc brakes.
The 14-bolt axle, also known as a GM Corporate, came in 3/4- and 1-ton trucks since the early 1970s and was produced under GM’s roof until 1994. Modern descendants of the axle can still be found under new trucks.
A quick shopping spree on RockAuto landed us all we needed to swap a 14-bolt from antiquated and heavy drum brakes to lighter, more-modern discs. (See the table for the exact parts required for the swap.)
The factory drums are over 15 inches across and fill the entire expanse of the wheel.
The 14-bolt’s inherent strength comes from the full-float design. The benefit of a full-floating axle is twofold. First, it prevents the wheel from coming off in the event of an axleshaft failure. Second, it allows for the use of very large bearings and a rigid cast iron wheel hub, to positively locate the wheel.
The first step is to remove the axleshaft, an easy job on a full-float design. Hit the eight exterior axle bolts with an impact gun or ratchet and the axle slides right out. Be warned that it will be coated in drippy, smelly gear lube because the 14-bolt is an oil-bath-lubricated axle.
Next, remove the snap ring and the keyway that keeps the hub nut from spinning.
Resist the screwdriver, chisel, and hammer! A special six-lug socket is needed to remove the wheel nut. Most auto parts stores carry it. The hub nut should unscrew with moderate force, though an impact gun is always handy.
With the hub nut removed, the entire hub and drum assembly can be removed.
With the drum/hub out of the way, remove the four bolts holding the drum’s backing plate to the axle housing.
The last thing holding the backing plate to the axle is the brake line, which can be removed with a 3/8-inch wrench.
Slide the backing plate off and Frisbee it into the nearest trash bin.
The wheel hubs will be reused but first need to be separated from the drum. This is done by pounding out the wheel studs. If you plan on reusing them, use a hammer and brass drift to knock them out to avoid damaging the studs. If you plan on adding new studs, just hammer away. Hoss. Note: If you will be running aluminum wheels, longer wheel studs (Dorman PN DOR-610-301) will be required.
Before setting the new brake rotor onto the hub, make sure to wire-wheel the mating surface, removing any debris or corrosion.
The rotor simply drops onto the back of the hub and is located by pressing in the new/reused wheel studs.
The completed rotor/hub assembly is ready to be reinstalled onto the spindle. If your axle had play or was leaking oil, now is the time to replace the bearings and/or install a fresh set of wheel seals.
Making the job so easy are these bolt-on brackets from Ballistic Fabrication. They’re stamped from thick, 3/8-inch steel plate and bolt in place of the drum backing plates, allowing a factory-style GM caliper to properly locate over the rotor.
With the brackets in place, reinstall the hub, opposite of removal, and slip the caliper over the rotor. The caliper is a floating style and adheres to the bracket by two Allen-head bolts.
To make pad changes easier down the line, we installed factory-style flexible brake hoses. These attach to the caliper via a banjo-bolt and crush washers (included with the calipers).
Two weld-on brackets were ordered from Summit Racing Equipment to allow the flexible hose and conventional hardline to meet. This keeps things from moving around on the trail and cleans up the installation. The axle was ground clean and a Miller 211 welder used to burn the brackets in place. Note our tabs weren’t compatible with the C-clips on the brake hose, but a 1/2-inch nut will do the job.
This brass bulkhead fitting from Earls (PN 972052ERL) bolts onto one of the differential cover bolts and splits the hydraulic line between bolt calipers. If you’re doing the swap on a running truck, the original bulkhead fitting can be reused.
Conventional 3/16-inch hard line with 3/8-24 inverted flare nuts spans the gap between the bulkhead fitting and brake hoses, completing the job. Now it’s time to wire-wheel and paint the rest of the axle housing––and maybe degrease the floor.
The grand total for our swap was $280 (includes $40 of core charged for the calipers). We left out the couple bucks’ worth of brake fittings since setups will differ and most axles will already have a bulkhead fitting, and in most cases tube nuts and that sort of thing can be reused.
|Calipers (front)||1977 Chevy C20||RockAuto||$93.58|
|Rotors (front)||1977 Chevy C20||RockAuto||$105.58|
|Caliper brackets||N/A||Ballistic Fabrication||$39.99|
|Brake pads (front)||1977 Chevy C20||RockAuto||$21.79|
|Brake hoses (front)||1977 Chevy C20||RockAuto||$15.12|
|Hose tab||Aftermarket||Summit Racing Equipment||$3.94|