The Ford 9-Inch Third Member Explained

A Ford 9-inch third member is the removable rear axle assembly that bolts into the rear axle housing of a Ford vehicle. The "third member" is the name for the internal gear set and differential — the part that actually splits engine power between the two rear wheels. Ford's 9-inch version, introduced in 1957, became one of the most common and durable rear axles in automotive history, and it remains popular in restoration, hot rod, and performance builds today because parts are widely available and the design is straightforward to modify.

The third member sits inside the axle housing and contains the ring gear, pinion gear, and differential spider gears. When you remove the cover plate on the back of the housing, you can see into the third member. The entire assembly can be unbolted and lifted out as one unit, which is why it's called a "member" — it's a distinct, replaceable component rather than something welded permanently into place.

Key Takeaways

  • The third member is the internal gear assembly inside the rear axle housing that distributes power to both rear wheels.
  • Ford's 9-inch third member has been in production since 1957 and is found in many Ford, Mercury, and Thunderbird models across multiple decades.
  • The 9-inch is popular in custom builds because the design is straightforward, parts are inexpensive and abundant, and it can handle significant horsepower with the right gearing.
  • Third members can be swapped between compatible housings, so you can upgrade or change gear ratios without replacing the entire rear axle.

Which Ford Vehicles Used the 9-Inch Third Member

The Ford 9-inch third member appeared in full-size Fords starting in 1957, and Ford continued using it through the 1970s and into the 1980s in some models. It was standard in Fairlanes, Torinos, Mustangs (particularly 1968 and later), Cougars, and Thunderbirds. Mercury used the same assembly in Cougars and Marauders. Ford also installed 9-inch third members in some light-duty trucks, though truck applications are less common than passenger cars.

The exact years and models vary by body style and engine size. A 1969 Mustang with a big-block engine, for example, would have a 9-inch, while a 1965 Mustang with a 200-cubic-inch inline six might have a smaller 8-inch. The best way to confirm what you have is to measure the diameter of the housing cover or look up your vehicle's original build sheet if available.

How to Identify a Ford 9-Inch Housing

The simplest way to identify a 9-inch is to measure the diameter of the round cover plate on the back of the axle housing. A 9-inch measures approximately 9 inches across. An 8-inch will measure about 8 inches. You can also look at the casting number on the housing itself — Ford stamped identification numbers on the passenger side of the housing, and online databases can cross-reference those numbers to confirm the year, model, and size.

Another clue is the bolt pattern on the cover. A 9-inch uses ten bolts arranged in a circle; an 8-inch uses eight. If you can see the back of the housing clearly, counting the bolts is the fastest check. The 9-inch is also slightly heavier and more robust-looking than smaller versions, which is one reason it became the standard for performance applications.

Why the 9-Inch Became Popular in Custom Builds

The Ford 9-inch third member dominates the hot rod and restoration market for three reasons: availability, simplicity, and strength. Tens of thousands were produced over three decades, so used housings and third members are inexpensive and straightforward to find at salvage yards, online marketplaces, and specialty suppliers. Parts like ring gears, pinion gears, and bearing kits are still manufactured by multiple aftermarket companies, keeping repair and upgrade costs low.

The internal design is also straightforward compared to some other differentials, which means rebuilding or modifying one does not require exotic tools or specialized knowledge. A competent mechanic with a basic shop can disassemble, inspect, and reassemble a 9-inch third member. Finally, the 9-inch can handle significant horsepower — 400, 500, or even 600 horses — when built with quality bearings, gears, and a locking or limited-slip differential. That combination of toughness, simplicity, and low cost made it the default choice for anyone building a performance car on a budget.

Gear Ratios and How They Affect Performance

The gear ratio inside the third member determines how many times the ring gear turns for each turn of the driveshaft. A 3.00:1 ratio means the ring gear spins three times for every one rotation of the driveshaft; a 4.11:1 means it spins 4.11 times. Lower ratios (like 2.79:1 or 3.00:1) favor highway cruising and fuel economy because the engine turns fewer revolutions per mile. Higher ratios (like 4.11:1 or 4.56:1) favor acceleration and low-end torque but increase engine RPM at highway speeds.

One advantage of the removable third member is that you can change the gear ratio by swapping in a different third member with different gears, without replacing the entire housing. If you have a 3.00:1 and want more acceleration, you can find a used 9-inch third member with a 3.70:1 or 4.11:1 ratio and bolt it in. This flexibility made the 9-inch attractive to people who wanted to experiment with different setups or adapt a car for different purposes — highway cruising one season, drag racing the next.

Limited-Slip and Locking Differentials in the 9-Inch

A standard open differential in a 9-inch third member allows each rear wheel to spin at a different speed, which is necessary for turning corners smoothly. However, an open differential can cause traction loss on slippery surfaces or during hard acceleration — if one wheel loses grip, the engine power goes to that wheel instead of the one with traction. A limited-slip differential uses clutch packs or cones to limit the speed difference between wheels, keeping more power at the wheel with grip.

A locking differential (or "locker") mechanically locks both wheels together so they must spin at the same speed. This maximizes traction but makes the car harder to steer during tight turns because the inside wheel cannot spin slower than the outside wheel. Many performance builds use a limited-slip as a compromise — better traction than open, but still drivable on the street. The 9-inch can be rebuilt with any of these differential types, and aftermarket suppliers sell complete assemblies ready to bolt in.

Swapping a 9-Inch Third Member Into a Different Housing

One reason the 9-inch is so versatile is that the third member itself can be removed and installed into different housings. If you have a car with a smaller or weaker rear axle and want to upgrade, you can source a 9-inch housing, have it welded or bolted to your car's frame, and then install a 9-inch third member inside it. This is common in older Mustangs, Camaros, and other classics that originally came with 8-inch or smaller axles.

The catch is that the housing must be the correct width for your car's track (the distance between the wheels). A 9-inch housing comes in different widths — typically 56 inches, 60 inches, or 65 inches — and you need to match the width to your car's frame and suspension geometry. Installing a housing that is too wide or too narrow will throw off the wheel alignment and handling. Shops that specialize in rear axle work can help you source the correct width and may support proper installation.

Frequently Asked Questions

Can I rebuild a Ford 9-inch third member myself?

Yes, if you have mechanical skill and basic tools. Disassembly requires a gear puller and a press, and reassembly requires careful attention to bearing preload and backlash. Many people send third members to a professional shop for rebuilding to may support the gears mesh correctly and the bearings are properly seated, which costs between $300 and $800 depending on what needs replacement.

What is the difference between a 9-inch and a 9-inch Traction-Lok?

Traction-Lok is Ford's name for a limited-slip differential. A standard 9-inch has an open differential; a 9-inch Traction-Lok has clutch packs that limit wheel slip. The Traction-Lok provides better traction during acceleration and on slippery surfaces but is more expensive and requires periodic maintenance of the clutch fluid.

How much horsepower can a Ford 9-inch handle?

A stock 9-inch third member with original gears and bearings can handle 400 to 500 horsepower reliably. Beyond that, you risk gear breakage or bearing failure. For serious performance builds, shops rebuild the third member with stronger gears, larger bearings, and a locking or limited-slip differential rated for 600+ horsepower.

Where can I find a used Ford 9-inch third member?

Salvage yards, online marketplaces like eBay and Craigslist, and specialty automotive suppliers all stock used 9-inch third members. Prices typically range from $100 to $400 depending on condition and whether the differential is open or limited-slip. Inspect the gears for wear or damage before purchasing, or buy from a seller with a return policy.

Do I need to change the driveshaft length if I swap in a 9-inch?

Possibly. If you are installing a 9-inch housing that is wider than your original axle, the driveshaft length may need adjustment. A driveshaft shop can measure your setup and shorten or lengthen the shaft as needed. This typically costs $150 to $300.