How to Identify a 10-Bolt, 8.5-inch Posi Rear Endby Vincent Basehart
When it comes to getting power to the pavement, GM's 10-bolt, 8.5-inch rear end with Positraction is a hero. Like Superman in street clothes, however, it can be hard to identify this beefy unit among its weaker cousins, the 10-bolt, 8.2-inch with open differential.
GM built countless thousands of both types of 10-bolts from 1970 to 1994. This means you can typically find one suitable for your muscle car dirt-cheap at your local junkyard. Learn how to separate this strong man from the wimps.
Wipe any grease and other debris off the gear casing with a rag to ensure you can see all of its features clearly.
Count to make sure there are 10 bolts arranged equally around the outer perimeter of the gear casing, similar to the numbers on a clock.
Locate two lugs, or ears, extending out from the bottom of the gear casing approximately in the 4 o'clock and 8 o'clock position. This will firmly identify the unit as having come with Positraction from the factory.
Measure the gear casing horizontally, end to end, across its widest point. The measurement will be either 10 5/8 (10.625) inches or 11 inches, depending on the year it was made.
Look for a bulge running vertically down the center of the gear casing. Most of the 10-inch units you're looking for will have this.
Place your 1.25-inch socket over the pinion nut. If it fits, it's the 10-bolt, 8.5-inch unit.
Inspect the entire gear case and center section thoroughly to make sure there are no cracks or other damage that might compromise the unit's integrity.
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