- Dan's Performance Page

How to build a better 10-bolt

I have always had a problem with the rear axle that came with my Trans Am: Known as RPO Code GT4, my original axle assembly came equipped with ten-inch rear rotors, coupled to 26-spline axles that were splined to an Eaton Guv-lock differential with a 7.5-inch 3.73:1 gearset. Now I imagine that this was an acceptable performance piece when the car was new, but with only 39,000 miles on the car, the clutches had already worn out. This becomes a problem with the Eaton unit, as the system will free-wheel like an open differential then lock solid with a bang. I lived with this malfunctioning differential for 90,000 miles! Since I was going to Gingerman raceway this year, the LAST thing I wanted to happen was an unexpected lockup of the differential. It upsets the whole suspension and will quickly send you into a spin. It usually happens in conjunction with a hard upshift.

Another problem with the GT4 is the original RPO J65 rear disk brakes: They have a faulty spring inside the piston that causes them to retract from the brake rotor. This results in excessive pedal travel, and no emergency brakes. Mine were corrected with a set of revised pistons from GM, but when I broke off a bleeder screw in the left caliper, I knew it was time to update.

My goal is to design a rear axle assembly that will hold up to about 400 horsepower. I looked at buying a 9-bolt, but parts are too expensive and hard to find. I looked at getting a Dana 44, but they were non-existent. A Ford 9-inch has alignment problems, and are easily $1500 for an entry-level assembly. What I settled on is the only solution that costs under a thousand dollars: A 10-bolt that uses the Gleason/Torsen heavy-duty Torsen differential, and aftermarket axles.

10-bolt Performance The Torsen heavy-duty differential is a unit that is currently being sold in the 1999 Pontiac SLP Firehawk. The LS-1 based Firehawk outputs about 345 rear-wheel horsepower, and offers a warranty to boot. Therefore, a goal for a 400-horsepower capable 10-bolt seems attainable with this differential. Research indicated that the Torsen is a direct bolt-in to any 3-series 10-bolt carrier. Thus, the Torsen should be able to fit in my 1984 GT4 axle housing. Unfortunately, the GT4, being a pre-1989 axle, uses 26 spline axles whereas the Torsen employs 28-spline axles. Therefore, a new set of axles are in order.

For these axles, I called Greg Moser of Moser Engineering. Moser Engineering can cut a set of 28-spline axles for the third-generation F-car which are much more capable of handling the high-torque demands of a performance application. Thus, the Moser axles were chosen in favor of stock 28-spline axles from a junkyard. There have been reports of broken OEM axles squirting out onto the racetrack and I want to avoid this possibility.

AAM Oil-cooling Differential Cover This cover has a patented oil cooler cast inside it. I saw a lexan version of this cover in action at the 1997 SAE International congress--- Very snazzy. It catches gear oil as it is flung off the ring gear, and channels it through passages in the differential cover to the holes on either side of the differential case: These holes, normally used for case spreading, open up into the axle tubes on the other side of the differential bearings. The oil is dumped into the axle tubes, where it flows both to the load bearing at the wheels, and back into the differential case. Thus, your axle tubes become a method with which excess heat is drawn out of the oil. The AAM cover is also nearly half an inch thick at the mounting flanges, which should greatly increase the rigidity of the axle assembly. This should further increase reliability of the gearing.

One note: Mike Galda has just informed me that SLP is now shipping a template that shows some grinding operations that you need to perform before installing the differential. You need to grind a drain-back path for the oil. Here's the template: drainback path
Troy ( has these suggestions on further increasing the strength of the 10-bolt:

Weld the axle tubes all the way around,where they go into the housing to increase strength of the housing. The factory only used 2 spot welds here.
Use Redline Synthetic shockproof geal oil.
Use a Zytanium cross pin. It is available in the year one next generation catalog for $28.
Use a solid bearing spacer instead of a crush sleeve. It costs about $20 from Ratech engineering
Troy runs a 1992 Trans Am GTA, and is using Richmond 3.42 gears, National Drivetrain 28-spline axles, and a Summit rear girdle, along with the heavy-duty Torsen differential. 
Installation of the Gears: Most people are scared of installing their own gears in a differential for fear that they will screw something up and blow the whole shebang. For good reason! After getting ripped off by a certain automatic transmission shop on a ring & pinion install, I got smart and found specialists who do gears every day: K&L Unlimited in Lansing, Michigan. K&L doesn't do automatic transmissions: They do gears. They also set up manual transmissions, and build custom driveshafts. They balance driveshafts. Pretty much any final drive piece is fair game. They don't have a lift, so you'll have to pop the parts out of your car yourself. Prices are reasonable, too! This gear installation cost me $192 in labor, which is (imho) very reasonable given their level of experience.
K&L Unlimited
2919 S. Martin Luther King Blvd.
Lansing, Michigan

Phone: (517)882-9101
Fax: (517)882-7544

The Ring & Pinion chosen are 7.625" 3.73:1 Richmond gearset. I bought these gears a few years ago, and already had them in the axle assembly. It was a question as to whether they would be any good after nearly 40,000 miles of driving, but they showed no appreciable wear. Therefore, we re-used them! Richmond 3.73 gears are rather noisy from the outset, but are strong.

The Torsen heavy-duty differential was purchased through SLP-Performance on their Internet specials page. SLP offers two different differentials: An OEM take-off version, and the heavy-duty version that they installed in the 1999 Firehawk. The OEM is a real bargain, being virtually brand new, and already sporting bearings. Bearings cost $25 or more per pair anyway, so this is a good deal. The heavy-duty version is brand new, without bearings, but comes with a cast aluminum differential cover from American Axle and Machining (AAM).

The Torsen differentials are designed for use in a 3-series carrier.

10-Bolt Update:
I had a conversation with Richmond engineers a few months ago on the reliability of their 7.5" ring & pinion.  I wanted to know if it could hold up to 400 horsepower and they said, essentially, no.  Actually, they said that 400 horsepower was "pushing it", but that a stud girdle would help greatly in preserving gear life. Richmond claims that the biggest flaw with the 10-bolt design is the flexibility of the case. When under heavy load, the pinion walks up the ring gear face, and forces the case apart. This totally throws off the gear alignment which causes premature gear failure. The key, according to Richmond, is to maintain alignment by increasing case rigidity. They claim that a stud girdle, like the one offered by Summit Racing equipment, or the TA Performance piece offered by Fast Toys, will "essentially double" the gear life of a differential used under high load conditions. A stud girdle is a very stiff cast aluminum differential cover, with studs that extend to the load bearing caps. It ties the rear of the case to the bearing cap surface, greatly increasing rigidity.

In any event, Richmond said that the useful power range of a 10-bolt stops at 400 horsepower. Further conversations with Steve Spohn along with Phillip Reddy concur with this assessment. They have both experienced failures when power increased above 350 HP, especially under launch with a modified suspension and drag racing slicks.

Related links:

Moser Engineering: Custom Axles

Want to see more? Click here to return
Dan's Third-gen Hobbyist page

If you have comments or suggestions, email me at