Ford 9 Inch Parts

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Rear Axle Family

Ford 9 Inch

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Rear Axle Family

Ford 9 Inch


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There is little argument that the venerable Ford 9-inch rear end is one of the most sought-after performance driveline parts for street rods, muscle cars, and race cars. The Ford 9-inch has been a mainstay of performance car builds no matter the vehicle manufacturer for over half a century. While some people may dislike a Chevy engine installed in a Ford, the 9-inch Ford rear end holds an extremely high acceptance level and can be found under everything from a ’40 Plymouth to a ’69 Camaro and a whole lot more! Why is that so? Well it comes down to the strength of the Ford 9-inch rear end design and the extremely generous aftermarket support for the Ford 9-inch rear end. If you are building a 9-inch Ford rear end for your project know you have plenty of options in gear ratios, axles, bearings, brake kits, and much more!

What Makes the 9-Inch so Strong?

In a nutshell, the distance between the ring gear and pinion gear, known as the hypoid distance, is where the Ford 9-inch gets its strength from. This increased distance provides for more gear tooth surface contact, especially in lower (numerically higher) gear ratios where there are fewer teeth on the pinion gear. This increased gear distance, along with the large ring gear diameter makes for a very strong Ford rear differential. The Ford 9-inch center section, also referred to as the Ford 9-inch third member, sees greater strength in the use of a bolt-in pinion support with a pair of tapered roller bearings to support the pinion gear, along with a third bearing that supports the nose of the pinion gear to reduce pinion gear deflection under load. The ability to pull the center section allows easy gear ratio changes to compensate for different tracks/usage without disturbing the ring and pinion gear setup. It is common for racers to have several different center sections with different gear ratios in their trailer ready to go. Lastly, we have the Ford 9-inch housing that the center section bolts into. The 9-inch housing uses large 3-inch axle tubes for strength and is designed for on-axle bearings.

How Hard is the Ford 9-inch to Retrofit?

Finding a useable complete Ford 9-inch rear end in a salvage yard is rare these days. While there is the occasional score, most Ford 9-inch rear ends going under projects these days are a combination of manufacturer’s parts or complete assemblies from one manufacturer. At Speedway Motors we offer the Ford 9-inch as a complete universal assembly (sold in various widths) ready for your suspension pickup points/tabs to be welded on, or as a complete bolt-in assembly for popular muscle cars/chassis. Obviously, the bolt-in solution is the easiest, but if you’re building something outside the scope of our bolt-in offerings, then it will be time to fire up the welder, or take the rear end housing and suspension pieces to your favorite welder or chassis shop. Of course we offer everything you need to build an existing Ford 9-inch rear end you may already own, or if you wish to mix-and-match parts to build the exact Ford 9-inch rear end you need.

What Gear Ratio Should I Choose?

The Ford 9-inch rear end has the largest selection of ring and pinion gears available than any other performance oriented rear end on the market. Popular Ford 9-inch ring and pinion gears are available from 3.00:1 to 6.50:1, allowing you to utilize the best ratio for your combination. Now determining the right ratio will take a little effort on your part. There are numerous gearing calculators on the Internet you can use, not to mention talking to your engine builder about where your peak power and torque come in, factoring transmission ratios into it, tire diameter, and more, which should all be taken into account when considering your Ford 9-inch gear ratios. As a rule, gear ratios from 3.50:1 to 3.90:1 range are what you would use on the street with a non-overdrive transmission. If you have an overdrive transmission, then you can jump into the 4.00:1 ratio up to 4.50:1 or so. Anything higher should really be best used for track applications only. Never fear though, if you need to change gear ratios you can easily swap your Ford 9-inch center section as a complete unit, or simply swap out the gears. Our installation kits and Ford’s ring gear backlash adjusting nut design make gear setup easy.

Is Axle Spline Count Important?

Unless it was fitted under a muscle car or truck, most of the Ford 9-inch rear ends were fitted with 28-spline axles. These are fine for a Sunday cruiser and mild performance builds (under 400 hp and no sticky tires), but if you are looking to build some real power to put to the ground you’ll want to procure your Ford 9-inch axles with increased spline counts. The popular 31-spline axle upgrade is still quite viable these days for street and performance use and will take hard launches on sticky tires. The increased spline count provides a greater overall surface area to engage with the differential side gears or spool. Just like the input shaft on a transmission, when you increase spline count you increase the strength of the connection between the splined parts due to the extra surface area the additional splines bring.

What Else do I Need to Know When Buying a Ford 9-inch

If you are buying a completely assembled Ford 9-inch rear end there is little left to add to your cart except perhaps gear oil and friction modifier (if using a clutch-based differential). However, if you are building your own from scratch or rebuilding an existing 9-inch rear end you already have, then do not forget to add a differential to your order. You have several designs to choose from on the Speedway Motors website, including friction clutch, helical gear, and ratcheting locker styles. We also carry spools for you straight line folks. If you’re pushing a lot of power through your Ford 9-inch rear end then consider welding in a back brace for strength, and if you’ll be doing the labor yourself then be sure to see all of the Ford 9-inch rear end tools and small parts we carry to help you build your Ford 9-inch rear end right the first time. Lastly, do not forget the brakes! We have several 9-inch rear end bolt-on brake kits, including drum and disc offerings. A Ford 9-inch disc brake conversion is quite popular, and we highly recommend it!

Updated By Mark Houlahan