Ford 9 Inch Third Member & Center Section Components

What Is a Ford 9 Inch Third Member?

A Ford 9 inch third member is the removable center section of a Ford 9 inch rear end. It holds the ring and pinion gears, the differential and the bearings in one cast case. Because it drops out as a unit, you can swap it without pulling the rear end from the car.

That design is a big reason racers choose the 9 inch. You can keep a spare third member with a different ratio or differential type and change it between rounds. The case bolts to the front of the rear end, so your rear axle housings and brakes stay put during the swap.

Building your own center section instead? You'll find differential carriers sold separately for a custom setup. For layout, dimensions and identification tips, see our Ford 9 inch rear end and axle guide.

How Do You Choose the Right Spline Count?

Choose the spline count that matches your axles, because the axle splines have to fit the side gears or spool inside the third member. This category offers 31-, 35- and 40-spline assemblies. If you're moving to a higher spline count, plan on new rear axles too.

A 31-spline third member typically suits street-driven cars and mild bracket cars. A 35-spline third member adds strength for more horsepower, stickier tires and harder launches. In this category, 35-spline options run from 3.50 to 4.86 ratios, with both spool and helical choices.

40-spline is the heavy-duty choice for high-horsepower race cars. Strange Engineering offers an Ultra case 3.812 with a 40-spline spool for that kind of load. Count the splines on your axles or check their specs before you order, since a mismatch means the axles won't seat.

Posi, Helical Limited-Slip or Spool: Which Differential Type?

Pick posi or helical limited-slip for a car that sees street miles, and a spool for a dedicated drag car. Each type handles wheel speed differently, which changes how the car launches and turns.

Here's how the three differential types compare:

• Posi (clutch-type limited-slip) uses clutch packs to drive both wheels while still letting them turn at different speeds in corners.
• Helical limited-slip uses gears instead of clutches, so there's no clutch pack to wear or service.
• Spool locks both axles together for equal power to each wheel, which helps launches but makes tight turns rough on the street.

For clutch-type options, look at Strange Engineering S-Series Trac-Lok units. There's also the JEGS Ford 9 Inch Posi Traction Third Member Assembly in 31-spline with 3.70 and 4.11 ratios. For a deeper comparison, read our guide on limited slip vs posi.

Nodular Iron, Aluminum or Ultra Case: Which Case Material?

Nodular iron fits most builds, aluminum saves weight, and an Ultra case handles the highest race loads. The case supports the pinion and carrier, so its rigidity affects how well the gears stay in mesh under power.

Here's how the case options in this category break down:

• Nodular iron cases resist flex, with options like the Strange Engineering Pro Iron Case and a Speedmaster 31-spline spool with 4.11 gears.
• Aluminum cases cut weight, with options like a Speedmaster 35-spline helical unit and the special-order Strange Engineering Pro Aluminum Case.
• Ultra cases from Strange Engineering target heavy race use, offered as a 3.250 with a 35-spline spool or a 3.812 with a 40-spline spool.

Speedmaster also offers HD cases in 31- and 35-spline, including spool and TorqueWorm limited-slip units. For street miles, nodular iron or an HD case is a practical start. Choose aluminum when weight matters most, and an Ultra case when power and tire size outgrow a standard case.

Which Ford 9 Inch Gear Ratio Fits Your Build?

Match your Ford 9 gear ratio to tire height, transmission and how you use the car. As a starting point, 3.00 to 3.50 suits street and highway driving, 3.70 to 4.11 works for street/strip, and 4.29 to 5.43 is drag territory.

Numerically lower ratios like 3.00 or 3.25 keep cruising RPM down, especially with an overdrive transmission. Ratios between 3.70 and 4.11 give you sharper launches while staying livable on the street. The 4.29 to 5.43 options, including Speedmaster TorqueWorm units at 4.56, 4.88 and 5.43, are geared for the strip.

Tall rear tires lower your effective ratio, so factor tire diameter into your choice. For formulas and charts, see our guide to choosing the correct rear gear ratio. Ready to finish the rest of the rear end? Shop All differentials and rear end components.