The Ford 8.8 Mustang Rear End

Adding a 31-spline axle to a Ford 8.8 rearend means you are increasing the strength of that axle by 30 percent (19.3 + 10.7 = 30 percent increase in strength).

Here is a comparison between the stock 28-spline 8.8 axle (left), a 30-spline 12-bolt axle (middle), and a 31-spline 9-inch Ford axle (right).

Here is a comparison between the stock 28-spline 8.8 axle (left), a 30-spline 12-bolt axle

Axles: Bigger is Better
To increase the spline count on an axle automatically means the diameter of the shaft itself must also increase. We found an interesting strength-comparison chart created by Mark Williams that uses a 30-spline GM axle as a standard. This gives you an idea of why bumping the 8.8 to a 31-spline axle is such a good idea.

In addition, most aftermarket axles use a stronger material than stock. The Ford OE axles are made of SAE 1055 or 1541 carbon steel that are induction-hardened, which means they are hardened only about 0.150-inch deep while the majority of the axle remains relatively soft. Not only do performance axles use a superior alloy steel, but they also benefit from a thicker wheel-mounting flange and a stronger design with a gentler radius between the wheel flange and the axleshaft.

Axle Widths
All rear-axle widths listed here use the axle-flange measurement (wheel-locating surface or drum-to-drum) to establish the overall width. The variable that affects overall width compared to housing width is the stand-off distance of the axle flange to the housing.

Many of these widths were obtained through various sources, so we cannot vouch for their complete accuracy. We recommend using these widths as guidelines and actually measure the housing you wish to replace because we have found slight variations even within housings of the same design. Other variables include the thickness of a disc-brake rotor hat versus a drum brake.

By Jeff Smith

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