Ford 9-Inch Housing Identification and Axle Measurement
Walk any automotive event and there’s a darn good chance that any hot rod or muscle car you peek under is going to be sitting on a Ford 9-inch housing. It may not be a stock or original Ford 9 inch rear end, but it carries the original’s dimensions and specs and for all intents and purposes they are still considered a Ford 9 inch.
Why is the Ford 9in rear end the “go to” rear axle for street and strip builds for over 70 years? The answer lies in several of the Ford 9-inch’s design aspects, starting with the Ford 9 inch gears. The large 9-inch diameter ring gear (where the axle derives its name from), combined with its offset to the pinion gear makes for a larger ring and pinion contact surface area, increasing strength. Additionally, the Ford 9-inch utilizes a removable center section, often referred to as the Ford 9-inch pumpkin, Ford 9-inch chunk, Ford 9 inch third member, or even a Ford 9-inch pig! This allows quick gear ratio changes to accommodate different tracks or updates to your vehicle’s horsepower and torque ratings. This not easily accomplished with integral carrier axles, like the GM 10- and 12-bolt. Finally, using bearing retainers that secure the axle at the flange, versus a C-clip style that secure the axles at the differential, means in the case of axle failure the axle flange and wheel will stay with the housing and not leave the vehicle, potentially causing brake failure and lack of control on the racing surface.
Ford 9 inch rear end identification is easy and can be accomplished with nothing more than a deep socket as a “go-no-go” gauge for the bottom two retaining nuts on the center section. If the socket fits (as shown in our photo below) it is a Ford 8-inch housing. The 9-inch housing’s center section will not allow the socket to clear, and you must remove the two bottom retaining nuts with a boxed end wrench.
However, the critical aspects of the rear axle are the axle flanges and axle lengths. You must confirm the flange version to be able to order the correct drum brake backing plates, disc brake conversion kit, or even the axle’s bearing retainer. Axle length is not always identical from side to side, as many 9-inch applications use an offset center, so you must measure the housing carefully to be able to order the proper axles.
Measuring for your 9-inch axles requires a few basic measurements to be taken. While we do offer cut-to-fit axles, you still need to ensure you have the proper measurements to be within the cut-to-fit area of the axle's splines. Measuring from axle flange to axle flange (measurement A in the diagram below) will give you total axle housing width. Measuring from the outside of the axle flange to the centerline of the pinion gear/nut will give you the correct axle length required (B and C measurements below). Note that most, but not all, applications will usually have one axle longer than the other due to an offset center section, so don't just measure one side and assume the other is the same.
Updated by Mark Houlahan
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Ford 9 Inch Gear-Style Posi Differential Third MembersView$962.99Compare