I'm re-gearing my '86 GMC pickup , and want to go with a limited slip unit . It's a stock 2.73:1 8.5" 10 bolt right now , and there seems to be some confusion as to what will fit . As far as I can determine any 8.5" 10 bolt will fit my differential housing , the only difference being the axle spline count . Everything I've found says I should have 28 splines in the 1986 . Anybody here that can verify ? I'm in no particular hurry , it looks like winter arrived here with a vengeance .
The 8.5" 10 bolt was used in many different models by every GM division, with the exception of Cadillac, during the 1970s and 1980s. The carriers and gears are easily interchangeable, and the original gear ratios range from 2.41:1 to 4.10:1. The 8.5" 10 bolt has two types of axles: 28 spline and
30 spline. The 28 spline axles were used until 1988, and the 30 spline axles were used from 1989 and up. Therefore, if your '86 GMC pickup has a stock 8.5" 10 bolt with 2.73:1 ratio, it should have 28 splines. You can verify this by counting the splines on the axle shaft or by checking the axle code on the right lower cover bolt.
If you want to go with a limited slip unit, you have several options. You can either find a Positraction carrier from another 8.5" 10 bolt with 28 splines, or you can upgrade to a 30 spline carrier and axles. You can also use an aftermarket locker, such as an Eaton E-Locker, which can switch between an open carrier and a fully locked differential. However, you will need a special bearing to use a 30 spline carrier or locker in your 8.5" housing. You can also change the gear ratio if you want, as long as it is above 2.73:1. You will need to adjust the pinion depth and backlash accordingly, and use new bearings and seals.
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