Is my 10-bolt an 8.5 or 8.6 before I order a TrueTrac and bearings?

Telling them apart: measure from the lower center cover bolt to its neighbors — about 3-1/4 in on a 7.5, 3-3/4 in on an 8.5/8.6. The 8.5 has large square lugs at 5 and 7 o'clock; the 7.5's are small. Confirm with the codes stamped on the axle tube.

More 8.5/8.6 details from Speedway: a 1-1/4 in pinion nut, left-hand-threaded 7/16-20 ring gear bolts, and a 1.625 in pinion shaft with 28 or 30 splines. Most use C-clip axles. The 8.5 and 8.6 are identical in design. Check the spline count before ordering parts.