The GM 10-bolt rearend isn't considered by many muscle car enthusiasts to be a high-performance axle, mostly because earlier examples built prior to 1973 had a smaller, weaker 8.2-inch ring gear which had a low tolerance for extra torque created by a more powerful, modified engine. Most guys just stepped up to a 12-bolt or a Ford 9-inch rear and called it a day. But when GM updated the 10-bolt design for 1973 model year vehicles (and some cars as early as 1971), they increased the ring gear size to 8.5 inches.
The 10-bolt rear with the improved 8.5-inch ring gear can be easily identified by the two large triangular cast-iron lugs on the bottom of the center section in combination with an 11-inch wide differential cover. With its beefy ring gear as the basis for a robust performance-oriented rearend assembly, many upgrades are available for the 8.5-inch 10-bolt, including differentials (ours has a Detroit TrueTrac), differential covers with bearing support (we have a Summit version), and axle upgrades. (For more on identifying and building a stronger 10-bolt 8.5-inch rear, click here and here.) Our '75 Chevy Laguna project is just one of over 7 million 1973-'77 GM A-bodies that possess the corporate 8.5-inch 10-bolt. They are literally everywhere (did we mention trucks?), and their strength, aftermarket support, and sheer numbers make them a great choice for a warmed-over street machine.
The upgraded 10-bolt axles can be found primarily on '73-81 F-bodies (Camaro/Firebird) and 1973-'77 A-bodies (Chevy Malibu/Laguna, Pontiac Lemans/Grand Am, Olds Cutlass, and Buick Century/Regal). Later on, they also became standard equipment on the famed Turbo Buick G-bodies (Grand National, Turbo T, and T-Type). Equipped with the 8.5-inch ring gear, the 10-bolt rear in these Turbo Buicks would easily run quarter-mile passes in the 10s, proving the strength of the design. Simply put, these later 10-bolt rearends are worth keeping.
Recently, while upgrading the stock drum brakes to Baer SS4 discs on our 1975 Chevy Laguna (Installing Baer SS4 Rear brakes), we discovered that our stock wheel bearings and axle seals were worn. The OE wheel bearing rides along a machined, hardened surface on the end of the axle, and over time the heat treating on the axle can wear down from heat and friction. This is known as galling or spalling, and can be seen in its early stages as a small overheated rough area. We noticed some of this wear on our axles, so we decided to affect a simple repair while the axles were out for the brake upgrade.
Fortunately, Timken makes a cool problem-solver replacement bearing that can dramatically extend the life expectancy of your aging 8.5-inch 10-bolt rear. It works by changing the location of the wear surface on the axle where the bearing rides, moving it outward towards the end of the axle on a virgin section of the machined, hardened surface. Being closer to the axle's end, the bearing actually gives the axle a little more support, and allows Timken to incorporate the axle seal into the bearing, doing away with a separate seal. Note that there are two versions of the Timken bearing/seal assembly—our '75 Laguna took the smaller one (part No. TRP1563TAV, $27.97 Summit price, 1.399-inch inner bore summitracing.com). If performing this upgrade yourself, we recommend you bring one of your axles to the parts store to prevent getting the wrong size. (The computer at the parts counter won't say what size the bearing is. The larger bearing is part number TRP1559TV, $28.97 Summit price, and has a larger 1.622-inch inner bore.)
Follow along as Dutch Miller of Baer Brakes shows you how to make the upgrade.