Why The Ford 9-Inch Rear End Is Still America's Favorite Axle Decades After Ford Quit Making It
Ford stopped making the 9-inch rear end in 1986, but aftermarket companies are still making parts for the axle, and racers still rip down tracks with it.
What makes the Ford 9-inch special is its construction. The Blue Oval overbuilt the thing. For example, the pinion, the gear connected to the driveshaft and shaped like a pine tree with the top lopped off, is supported by three bearings. This belt-and-suspenders approach helps to keep everything where it needs to be to prevent breakage.
Also, the ring gear, which translates the pinion's lateral rotation 90 degrees to the rear wheels, is 9 inches in diameter, hence the name. The large diameter of ring gear, as well as how low the pinion sits relative to the centerline of the ring gear, lets the gear teeth make greater contact with each other, which is key for withstanding horsepower. For example, the rear end in a 6-cylinder first-generation Mustang had a 7.25-inch ring gear, while Mustangs with the high-output 289-cubic-inch V8 came with the 9-inch rear.
Another aspect of the 9-inch that racers love is its detachable third member. Unlike a lot of domestic rear ends, the center section that holds the pinion, ring gear, and differential comes out as one unit. This makes gear ratios easily swappable at a race track, as a racer can have multiple center sections set up for different conditions. (The differential is another set of gears that let the wheels move at different speeds without binding. This old-timey video is one of the best explanations of how a differential works.)
Finally, the 9-inch is easy to retrofit into vehicles that weren't originally equipped. The sheet-metal center section housing is easier to weld than cast-iron rear ends — useful for attaching different suspension pickup points. Plus, the axle housings come in a variety of lengths and are relatively easy to shorten, making them easy to fit under most cars.
