Strengthening The Rearend From An LS1 Firebird Formula - Maximum-Effort Fourth-Gen10-Bolt

Having originated with the Chevy Monza and then migrating to the Chevy S-10, the 7.625-inch 10-bolt sitting under your Fourth-Generation Firebird certainly lacks a performance pedigree. Spend one minute on any of the late-model forums dedicated to GM Performance and when the conversation turns to rearends, a group roar erupts: "Junk the 10-bolt. Swap in a Ford 9-inch or GM 12-bolt."

But hold on a minute; does the name Jim Jones and Jonestown ring a bell? That's right, 913 innocent souls and Jim Jones drank spiked punch and perished in 1978 as part of the cult called the People's Temple. If one of those lost souls had an original thought, perhaps their fate would have been different.

Let's state categorically that the 10-bolt isn't junk, and if built correctly and used for everything but extreme drag racing, it's a very reliable rearend.

According to Jason Bailey, owner of Texas Drivetrain Performance in Fort Worth, Texas, "In the stock form, supplied with a Torsen limited-slip differential, and a 2.73 or 3.23 gear for the automatic or 3.42 for the manual, the 10-bolt rearend will usually support up to 350 rwhp. With elevated power levels, the weak points of the differential start to show up, resulting in parts breakage.

"The 10-bolts' weaknesses are the relatively light weight of the housing itself along with the 28-spline axles. Housing flex allows the ring gear to 'walk up' the pinion with elevated power levels, resulting in a broken gear set or damaged differential. The metal used in the axles can twist, especially at the axle ends, shearing them off.

Here is the Yukon ring-and-pinion gear set from Randy's Ring & Pinion (PN GM 7.5-342) for $196.35. Yukon uses American Axle Manufacturing (AAM) gears as one of its gear suppliers. AAM is the OEM GM supplier of gears, and according to Jason Bailey, "They are the strongest and quietest gears on the market for the 7.5- and 7.625-inch 10-bolt."

"By addressing the weak points and strengthening the rearend, a hobbyist can have a 10-bolt prepared that will live up to and beyond 425 rwhp. I have many customer's cars that have dipped into the high 10s or low 11s as well as competing on some of the most grueling road race courses across the country. For the quarter-mile racecar, there are better choices, especially for cars pushing over 450 rwhp or shocking the drivetrain with high-rpm manual-transmission launches."

For higher horsepower applications, there are several bolt-in aftermarket rearends, including GM 12-bolts and the Ford 9-inch. Before spending over $2,000 on an aftermarket rearend, determine how the Pontiac will be utilized and then make an informed spending decision.

Follow along as we go through a 7.625-inch 10-bolt teardown and buildup. Although we don't cover each and every step of the process, this story will provide comprehensive coverage of the major steps and parts in order to take your 10-bolt to the next level. This rearend is destined for none other than the STS rear-turbo-equipped '00 Formula WS6 covered in earlier issues.

Before beginning the build, a suitable fully loaded rearend from a '98-'02 Pontiac F-body was sourced from a local junkyard for $300. Although it isn't pretty, it's the correct housing for the '00 Formula that is set up for the three-channel (ABS) versus four-channel (ABS and traction control) sensors. To tell if the rearend is a three- or four-channel, check for an electrical connector in the top/center of the differential housing; if so, it's a three-channel. Jason Bailey of Texas Drivetrain Performance set the rear up on jackstands and placed a drain pan under the rear differential cover.	Starting with the bottom bolts, each of the 10 bolts that hold the cover in place were removed with a 1/2-inch socket. Once the gasket seal is loose, the fluid begins draining. To break the cover loose, a wide screwdriver and a ball-peen hammer were used to tap on the sealing surface if necessary.	In this case, the differential cover came off without fanfare. Before removing any more components, the backside of the cover was examined. A large circular magnet is attached to the cover and any loose metal resulting from wear accumulates on the magnet in the form of paste-like metal shavings. Residue caught on the magnet was wiped onto a paper towel and inspected. All rearends have some wear, and in this situation, there were only minor amounts of shavings. Had any chunks of metal been discovered, it would indicate major problems, indicative of an imminent failure.
In order to remove the axles, the C-clip retainer bolt was loosened with a 5/16-inch wrench.	The bolt was removed to allow removal of the C-clip retaining block from the differential. On many clutch-type differentials, a cross-shaft is utilized in place of the C-clip retaining block. Regardless of design type, the function of the mechanical device is to positively locate the axles in the carrier.	By pushing in the axles and reaching into the differential, the C-clips were removed.