How to Identify a 10-Bolt, 8.5-inch Posi Rear End

by Vincent Basehart
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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.

Step 1

Wipe any grease and other debris off the gear casing with a rag to ensure you can see all of its features clearly.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

Place your 1.25-inch socket over the pinion nut. If it fits, it's the 10-bolt, 8.5-inch unit.

Step 7

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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