Chevy Chevelle Rear Axle Swap - The Great Rear Axle Comparo

Rear Axle Specs
Here's a case for bigger is better, at least in terms of strength. Larger-diameter ring gears, axles, and pinion shafts all constitute stronger components. Basically, the greater the spline count, the larger the shaft minor diameter, which is the diameter of the shaft at the base of the splines.

One big strength advantage of the Dana/S60 is its 35-spline axles that measure 1.50 inches in outside diameter at the splines. Compared to the spindly, 30-spline 12-bolt axles, the S60 axles are massive.

One big strength advantage of the Dana/S60 is its 35-spline axles that measure 1.50 inches

The weakest link in most stock rear axle assemblies (with the exception of the Dana 60) is most often the axle spline count. In this chart, you can see that the stock Ford 9-inch and 8.8 axles are the smallest, which is where you can easily increase strength by upgrading to 31- or 33-spline axles. Moser offers a simple formula for determining torque capacity. Just plug in your engine's peak torque number and do the math: engine torque x first gear ratio x rear gear ratio x gear efficiency = torque to both axles. 500 lb-ft x 2.48 x 4.10 x 0.90 = 4,575 lb-ft total torque.

The following chart from Moser lists the torque capacity for one axle and recommends choosing the smallest spline count that will do the job. Since torque is applied to both axles, choosing the smallest 30-spline axle for the above example means we have a total load capacity of 12,400 lb-ft, which offers an excellent margin of safety.

By Jeff Smith

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