Reference Manual

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Chapter 3: Axles: 7.5 -> 8.5 swap

A 8.5" rear axle from a '84 H/O, 442 or GN is a very simple bolt-in replacement for a 7.5" rear axle that offers the benefits of an improved rear axle ratio and increased strength.

At first glance the two axle assemblies above may look identical. In fact the foremost assembly is a 7.5" axle and the assembly in the background is a 8.5" axle. Aside from that, they are identical. The same upper and lower control arms are used, the brake parts are identical. In fact the only change required to swap an 8.5" axle in place of a 7.5" axle is a 0.99" shorter driveshaft ( OK, call it one inch).

Courtesy of Steve Ochs:

For all applications EXCEPT as listed below:
2.50" O.D. x 0.065" Wall Thickness x 52.42" long
Pink-Pink


For Monte Carlo SS ONLY (H.O. V-8):
2.75" O.D. x 0.065" Wall Thickness x 52.42" long
Brown-Green

For Hurst Olds/442 and Turbo Regal/Grand National (8.5" Ring Gear):
2.75" O.D. x 0.065" Wall Thickness x 51.43" long
Purple-Green

All lengths are from center to center of u-joints.

Colors are the color of the crayon stripes on O.D. of tube near middle of shaft

Of note is the shorter driveshaft used for the 8.5", 0.99" to be exact, also note that there is no mention of transmissions. All factory installed transmissions, THM200C, THM250(C), THM350C and the THM200-4r all used the same length driveshaft.