The Perfect 10

June 28th, 2011

Text and Photos by Marcel Venable

Building a Stronger 20-Bolt GM Rear Differential

Growing up in an automotive environment, I always heard how the GM 12-bolt rear differential was the superior model over the GM 10-bolt model.  When I asked why, most of my mentors treated my question as if it were a line from the Spinal Tap. “They have 12 bolts and not 10, so they’re better.”

As my curiosity grew, another question that I’d ask was why some trucks only laid down rubber from the passenger tire, and some left rubber from both tires when doing a burnout. Most of the time when I asked about this accelerated tire warring dilemma the response would be that a Posi or locker was the culprit, and “they’re loud when you turn corners.”

As the years have gone by, my search for GM 12 bolts has become more difficult. Plus, some of the cons that plague 12 bolts are the fact that they are mainly found in large GM cars, or ¾-ton trucks, which require a lengthy list of modifications to convert the axles, brakes and lug patterns over to a ½-ton truck bolt pattern, thus allowing for a wider range of wheel selection. Also, some Posis and lockers are loud and even down right annoying when driving around sharp corners.

We started here with a fresh and clean 10-bolt housing, which was powder coated candy apple red by the guys at Ludikrs Kustomz. John set the housing on his diff jig to make things easy to install.

During my search for a good rear diff for my most recent project, I was coming up empty trying to locate a 12-bolt rear diff. This got me thinking about what’s so wrong with 10 bolts. Just because the other diffs have two more bolts can’t mean that they’re better, right? Besides, ½ tons came equipped from GM with 10 bolts diffs, so what’s the trick for building a good one?

I’m going to come clean and say that I’m a big believer in the phrase to be old and wise, you must first be young and stupid, which was exactly what I have been throughout my years thinking that 10-bolt rearends were junk, and a Posi or locker is loud and hard to drive on the street.

Eaton/ Detroit Locker’s Truetrac carrier comes fully assembled, minus the ring gear. We chose to go with a 3:73 ring and pinion set to turn the 29-inch-tall tires with a turbo 700R transmission. Eaton/Detroit Locker recommends that a certified driveline mechanic perform the installation because the tools of the trade are required to perform the installation are very specific. We’ll walk you through the steps that it takes, but consult a professional or view the owner’s manual online at for more information on how to install the unit properly.

It was only until I stopped by J&S Gear in Huntington Beach, California, that this age old debate was settled. John Coulman, who has been building thousands of rear differentials for more than 20 years, including 12 bolts, 10 bolts, 8-inch/9-inch Fords, Danas—well, you get the idea here, kids, he’s an expert.

John also went on to tell me that there is a diff for all applications, and they all offer specific advantages, but he did put to rest the idea that 10-bolt diffs are junk. As a matter of fact, the ring and pinion gears are larger in a 10 bolt and are less likely to break if the installer sets the proper tolerances and the operator maintains the differential. He went on to point out that Eaton (the corporation that owns Detroit Locker) manufactures many other great locking traction improving carrier units.

For example, the open carrier is the type of unit that the factory installed in the 10 bolts. They only apply torque to the passenger tire for traction, which works well at best. Now let’s take a look at one of the more popular units that Detroit Locker manufactures, the NOSPIN unit. Even though they have been called a Posi or locker, NOSPIN carriers apply torque to both rear tires, improving traction greatly. When one wheel passes over an obstruction, it must travel a greater distance, and therefore move faster than the other wheel. When this occurs, the locker automatically allows for the necessary difference in wheel speed. During a turn, the inside-driven clutch remains completely engaged with the inside spider gear, and continues to drive the vehicle. The outside-driven clutch automatically disengages from the outside spider gear, allowing the outer wheel to move freely in the turn. When the vehicle completes the turn, the outside-driven clutch automatically reengages with the outside spider gear, as both wheels again travel at the same speed.

This has been a time-honored design for the Eaton Corporation, and it’s used in thousands of applications. They do, however, have the reputation for being a little loud during the ratcheting process while cornering, plus the clutches and springs need servicing during heavy use, which make the NOSPIN the perfect carrier for off-road and/or racing vehicles.

John knew that we were building a truck for the street and recommended Eaton’s product designed for applications that are street driven. Eaton’s Truetrac carrier features a proven helical gear design, eliminating the need for wearable parts, resulting in maintenance-free traction recognized not only for its toughness, but its smooth and quiet operation too. Sometimes referred to as the“quiet locker,” the Truetrac’s design features precision-forged gears that are designed to mesh perfectly, providing improved strength and durability over a standard-cut gear.

So what did I learn? That 10 bolts can be just as cool as 12 bolts, and lockers don’t have to bark when going around corners. Well, that’s not the only thing that the gear guru John Coulman is about. Take a look at how he installed the unit in our ’85 C-10 project, and check out the source box for information about the parts that he installed and where to get them.


Eaton/ Detroit Truetrac (http://www NULL.eaton

Classic Performance Products (Brakes) (http://www NULL.classicperform

PML (Differential Cover) (http://www NULL.yourcovers

Royal Purple (Oil) (http://www NULL.royalpurple

Ludikrs Kustomz (Powder Coating) (http://www NULL.ludikrskustomz

IFS Coatings (Powder Coating Supplies) (http://www NULL.ifscoatings

Superior Sandblasting (Sandblaster

Dept. STTR

8315 Beech Ave.

Fontana, CA 92335


J&S Gear (Installation/Ring and Pinion Set)

Dept. STTR

18222 Gothard St. #A

Huntington Beach, CA 92648


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