How To Make a GM 10-Bolt Axle Live Under Fire

By Jeff Smith (Photography: Jeff Smith)

Drag racing is all about overkill: Too much horsepower is never enough. The drivetrain takes all the gaff, so racers tend to it with Brooklyn Bridge-like structures to keep from scattering parts all over the dragstrip. The reality for budget racers is making the less glamorous stuff work.

With that in mind, we’re here to tell you that ordinary production parts can be made to withstand an avalanche of abuse. Dave Henninger is a classic example of a low budget racer who has discovered that an infusion of the right pieces will make the lowly GM 10-bolt as hail and hearty as Doktor Frankenstein’s monster.

Henninger races a ’71 Camaro in NMCA’s Super Street class, burning up the quarter-mile with 8.70 elapsed times and speeds in the mid-150s. While that may not be newsworthy in itself, the car runs those times with slapper bars and a 10-bolt rear axle! That’s impressive. Even more astounding, Henninger confides, “I’ve easily got over a thousand passes on this same rearend, and it’s never broken a gear or burned a bearing.” Henninger’s got a system that works. One of the keys to success, according to our intrepid racer, is a 3,100-lb limit. His package weighs 3,070 lbs with driver.

We queried Henninger and engine builder Kurt Urban on how to make a meager 10-bolt survive behind a 930hp small-block on nitrous. According to Urban, ‘There are a lot of little things, but no matter what else you do, you have to stage the car right.” Hence Henninger always preloads the gears on the starting line. He stages the car with the foot brake, engages the trans brake, then releases the foot brake. This “pulls” the teeth of the pinion gear and the ring gear together, eliminating the “free space” between them so that the gears mesh long before the Sherman-tank torque slams them together.

Like many racers, Henninger is a weight freak on a tight budget. The light-duty 10-bolt was original equipment on the Camaro. Though it weighs slightly less than the 12-bolt, the diameter of the pinion shaft is the same. Furthermore, high performance parts for the 10-bolt are in abundant supply.

Henninger inspects the housing for straightness and to make sure there’s no rust where the axle tubes join the iron centersection. To test the housing for straightness in his car, Henninger sets it up on a four-wheel alignment rack. Once he is certain that the housing is straight and sound, he welds the tubes to the centersection.

All 10- and 12-bolt housing tubes are plug-welded to the centersection in two places. Henninger augments the scheme by drilling six more holes on each side of the centersection, then welds them up. Next, he takes the assembly to a professional welder to have the tubes welded to the housing. (A professional knows how to quench the hot areas to prevent warpage.) In this case, Henninger chose to stay with the original housing ends rather than replace them with the larger, more common, early Oldsmobile pieces available from all major axle companies.

Mark Williams supplied the 35-spline axles, spool, and C-clip eliminator kit. Henninger sets up his own rearends, preferring Richmond gears as obtained from Reider Racing. In this particular application, he runs 4.56:1 cogs and sets the pinion at the factory depth. When new gears are installed, he sets the backlash tighter than the factory spec of .008 inch, opting for .0045-.005 inch.

Not the rearend or suspension you’d expect to find in an 8-second quarter-mile car, is it? Though difficult to see in this photo, both axle tubes are welded to the centersection to prevent them from twisting in the housing.

If you’re serious about horsepower, you must prepare the rear axle for abuse. The pinion shaft on the 8.5-inch 10-bolt has a diameter of 1.625 inches, the same as the GM 12-bolt. The diameter of the 12-bolt ring gear—at 8.875 inches—is .375 inch larger than the one in the 8.5-inch 10-bolt.

Then he checks the pattern for maximum tooth contact on the drive side of the gear. Henninger says that after a few passes, the lash on a new set of gears increases to about .007 inch. There is no special break-in procedure, but he likes to drive the car 5 or 10 miles, and then check the lube level, since some of it will have migrated into the axle tubes.

Do your homework and find the strongest and least expensive used 10-bolt housing that will work in your car. Then strengthen it with some simple, inexpensive welding, and invest in a few quality aftermarket parts to ensure long life. No unobtanium, no voodoo, and no incantations—just common sense and attention to detail.

You can apply the same logic to a daily driver. If you substitute a Posi for the spool, do the housing tune-up, and set the gears up properly, you can make a 10-bolt (or a 12-bolt) viable under the most extreme conditions. If Henninger can run 8.70s at 155 mph hundreds of times, your own 8.5-inch 10-bolt just might go to the grave with you.

Both Posi-traction and spool are available for the biggest 10-bolt. A clutch-type Posi is good for the street, but for ultimate duty, the spool rules.

A properly set up ring-and-pinion is the key to longevity. Henninger adjusts for maximum tooth contact on the drive side of the ring gear, as illustrated by this ideal pattern. Henninger sets backlash for a new set of gears at .0045-.005 inch.

Tough axles are a must with the 10-bolt. Mark Williams, Strange, Moser, and others make high quality axle shafts for either strip or street duty. A C-clip eliminator kit is both mandatory for safety and required by the NHRA for cars that are as quick as a Super Streeter.

The driver-side cast iron bearing cap represents potential for failure. LPW offers a cast aluminum rear cover incorporating a preload bolt that supports the cap to prevent deflection.

T&D Machine offers this quality pinion depth tool, the most accurate way to do the setup

Dave Henninger’s 71 Camaro will never win a beauty contest, but it’s damn quick. It evolved from a street racer to win the first NMCA Super Street class race at Memphis in 1993 with a 9.50 e.t. at 142.5 mph.

The car sports a 400ci small-block with a Precision steel crank, Bill Miller aluminum rods, 11.0:1 pistons, and a set of Jim Tarian-ported Dart 220 heads. The valves are tickled with a 278/288 duration (at .050 inch) and Comp Cams grind, while the junk is excised via Stahl headers and Flowmaster mufflers. The trickest part of the whole package is the Holley tunnel ram intake manifold and BDS bug catcher, which has been plumbed with a sequential DFI fuel injection system. With a Kurt Urban tune-up on the fuel, spark, and nitrous curves, this Camaro runs consistently, with a minimum of breakage.

Henninger plans to try his hand at Real Street in 1997. A single butterfly in the BDS hat will limit the engine to the same throttle bore size as a 750-cfm carburetor. Urban will rebuild the engine with a 14.0:1 compression ratio, but all the while that 10-bolt makes like Gibraltar.

Henninger’s Camaro doesn’t look like anything special, but it sure flies. He plans on building a smaller engine, making some minor changes, and running without nitrous oxide in Real Street class for 1997.

A key to Henninger’s performance in Super Street is the DFI electronic fuel injection. He uses a Holley tunnel ram under the BDS injector hat in combination with nitrous oxide. On juice, the motor makes 930 hp at 7,800 rpm. Nitrous fuel enrichment is handled by the fuel injectors.

Not all GM 10-bolts are created equal. The key to longevity is finding the right axle housing. There are several different styles. The one with the 7.5-?nch ring gear, common to the ’82-’94 Camaro and Firebird, is the weakest of the lot. The ’64-72 10-bolt in the Camaro, Chevelle, and Nova measures 8.2 inches across the ring gear, so stay away from that one too. The 8.5-inch version is strongest, and it’s indigenous to a variety of GM models (70-’81 Camaro, GTO, and Firebird, 70-’75 Chevy II, and 70-’76 Chevelle being the most plentiful).

According to Henninger, the easiest way to identify the preferred 8.5-inch 10-bolt is when it’s equipped with a limited-slip carrier. This version uses a clutch-type Posi-traction, while those with a smaller ring gear use a cone-style unit. Other than measuring the ring-gear diameter, there’s no easy way to tell them apart if they’re equipped with an open differential.

The following chart lists the year and model of cars with an 8.5-inch 10-bolt. Many of these applications will fit older models. For example, a 70-72 Chevelle 10-bolt will go right under a ’64-’67 Chevelle, even though the later housing is slightly wider than the one it replaces.

The following companies offer high performance parts for the GM 8.5-inch ring gear 10-bolt.

California Gear, Division of Chrisman Racing
Dept. HR05,1321 S. Lewis St., Anaheim, CA
92805.714/776-2501

LPW Racing Products
Dept. HR05,632 E. Marion St., Lancaster, PA
17602. 717/394-7432

Mark Williams Enterprises
Dept. HR05,765 S. Pierce Ave., Louisville,
CO 80027,303/665-6901

Moser Engineering
Dept. HR05,1616 N. Franklin, Portland. IN
47371, 219/726-6689

Precision Gear
Dept. HR05. 12351 Universal Dr., Taylor, MI
48180,800/522-2707

Strange Engineering
Dept. HR05,1611 Church St., Evanston, IL
60201,847/869-7010

T&D Machine Products
Dept. HR05, 4859 Convair Dr., Carson City,
NV 89706, 702/884-2292

Tom’s Differentials
Dept. HR05, 15709 Paramount Blvd.,
Paramount, CA 90723,310/634-8431

MOTORTREND and HOT ROD’s rich magazine history and legacy dating back to 1948 is something highly valued by its longtime readers, and that’s why we’ve invested deeply to make the content available to you in a modern and accessible format. In the interest of transparency, these magazine articles are presented as originally published, without modification, and may contain content that does not reflect the company’s contemporary values and standards.