Everything You Need To Know About a Ford 9 Inch Rear End

Our 1957 Ford Ranch Wagon Gets Rear End Components From Strange Engineering

By Ron Ceridono – Photography by Tate Radford

If you’ve been following the construction of Colin and Sue Radford’s ’57 Ford Del Rio Ranch Wagon so far you’ve seen the installation of the Art Morrison Enterprises front bikini clip, a Ford Performance 5.0 Coyote Engine and Art Morrison Enterprises an four-bar kit with their 9 inch rear end housings. This time around we will get into the whys and wherefores of selecting the components from Strange Engineering to make the rear end complete.

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02 Our new Strange nodular iron rear end came with a Daytona nodular iron pinion — Our new Strange nodular iron rear end came with a “Daytona” nodular iron pinion support and a heavy-duty driveshaft yoke to accept a 1350 series universal joint.

What Years Did Ford Use The 9 Inch Rear End?

Ford’s 9 inch rear end was produced from 1957-86 (the 9-inch reference refers to the diameter of the ring gear). So, yes, our wagon did come with a 9-inch, so why did we change it? There were a number of reasons for the decision. The new housing came from AME with all the necessary brackets for the four-bar and Panhard bar welded on in a fixture, so we knew they were located correctly and didn’t have to worry about the warping from the original housing welding those items in place in our shop. It also came with the late, big bearing, Torino-style axle ends that provide more options when selecting aftermarket disc brakes.

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03 Strange differential assemblies are available with a variety of gear ratios — Strange differential assemblies are available with a variety of gear ratios. We chose a 4.11 ring-and-pinion as the wagon will be equipped with an overdrive automatic, tall rear tires, and will be towing a restored camp trailer from time to time.

9 Inch Rear End Differences

We should point out there are three distinct 9-inch axle ends: the late big bearing (Torino) and the early big bearing both take a 3.150-inch od wheel bearing (the difference is in the bolt spacing for the backing plates), the small bearing ends take a 2.834 od bearing with yet a different backing plate bolt pattern. Our final reason for making the change is that the custom width housing will allow us to fit the wheels and tires we have planned without modifying the fenders or wheel wells—all things considered a new housing was the practical choice.

04 Properly setting up a ring and pinion is practically an art form — Properly setting up a ring-and-pinion is practically an art form. The technicians at Strange have the procedure down. Here a dial indicator is being used to determine the backlash, or the play between the gears.

One might think that since the 9-inch went out of production when Ronald Reagan was president parts might be hard to find. But everything from individual parts to complete rear ends are readily available from Strange. They knew a good thing when they saw it and realized the advantages to the design of the 9-inch Ford rearend and made some improvements to make a good thing better.

05 This is a standard open differential — This is a standard open differential. Arrow A points to the side gears, arrow B points to the spider gears. When going straight the spider gears don’t spin (note the alignment marks) and the side gears turn at the same speed.

Why Is The Ford 9 Inch Rear End Popular?

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Referred to as the Hotchkiss style, the case containing the third member, or the ring-and-pinion assembly, of the 9-inch is removable as a unit. By comparison the Salisbury design, like the later Ford 8.8 and others, have the third member components mounted in the axle housing, making gear installation and adjustment much more complicated. Another important issue is the method of retaining the axles. The 9-inch uses pressed-on bearings and lock rings on the axles, which are held in the housing with retainers bolted to the flanges on the axle tubes. If an axle should break the wheel will stay attached to the housing. By comparison, the Ford 8.8 and GM 10/12 bolt axles are held in the housing by C-clips inside the differential case. That means a broken axle, along with the wheel and tire, can come completely off the car. (Strange does offer C-clip eliminator kits that resolve that problem.)

Open Differential

One of the components found in production cars is some sort of differential. When a car turns a corner the outside drive wheel travels in a larger circle than the inside wheel. As a result, the outside wheel must turn faster—it’s the differential that allows this to happen. The most common type of differential is the “open” style. The problem with this design, as editor Brian Brennan learned early on in his mom’s car, is if one tire loses traction the differential will transfer all the power to that wheel (young Brennan was known for one-tire smoky burnouts).

07 We opted to include a Strange clutch style limited slip differential in the wagons rear — We opted to include a Strange clutch-style limited-slip differential in the wagon’s rear; the forged steel housing eliminates breakage common to cast OEM designs. Note the perfect contact pattern on the ring gear that was established when the gears were installed.

Welded Differential

While a differential is obviously necessary in a street-driven car, for racing applications delivering power to both wheels is certainly an advantage. In the early days some racers simply welded the differential spider and side gears into a solid mass to drive both wheels—crude, but effective. Today a much more reliable device called a spool is used to eliminate the differential—but they should never be used on the street, something that will become obvious when turning a corner in a car equipped with one.

When it comes to performance differentials there are those who will put power to the ground from both tires. Strange offers a variety of such designs and explains them as follows.

08 This is a typical clutch style limited slip unit the clutch plates supply power to both wheels but slip when necessary — This is a typical clutch-style limited slip unit, the clutch plates supply power to both wheels but slip when necessary (during cornering). Note it still uses standard differential spider gears (arrows).

Clutch Type Differential

“Clutch-type differentials use a system of clutches (sometimes on one side, sometimes on both sides) to distribute pulling force between the two axles. Spider gears are also used in this design.

09 A popular limited slip differential available from Strange is the helical gear style — A popular limited-slip differential available from Strange is the helical gear style. They cause both axles to pull under acceleration and still operate as a differential going around corners. They are strong and well suited to high-performance applications.

Cone Type Differential

A tapered housing with cone-shaped gears gives the cone-type differential its name. It works in much the same fashion as the clutch-type differential but uses spring pressure instead of clutches.

10 Helical limited slip units have clutch plates that are squeezed together under acceleration to drive both wheels — Helical limited-slip units have clutch plates that are squeezed together under acceleration to drive both wheels. They are unique in that they don’t have differential side gears, or spiders, like an open differential or clutch-type posi.

Locking & Helical Differentials

For a truly high-performance application, lockers and helical gear units accomplish the same fundamental purpose–they cause both axles to pull (only) under acceleration. Differentials like the Detroit Locker are very strong but can be noisy. Helical differentials offer a smoother transition along with a level of durability comparable to that of the locker.”

11 Detroit Lockers are rugged and provide the ultimate in traction — Detroit Lockers are rugged and provide the ultimate in traction, but they can be harsh in operation and even make short wheelbase cars seem twitchy in corners. In operation the projections on the hubs and the corresponding slots they fit in lock the axles together.

What Is The Best Ford 9 Inch Rear End?

At one time the most sought-after factory differential case for building a truly tough 9 inch rear end was the nodular iron version found in high-performance Fords like the Cobra Jet and BOSS 302 Mustang, even some pickups. Original nodular iron cases are hard to find, but why bother as Strange Engineering offers their nodular iron S case that is stronger than the originals due to better materials and enhanced ribbing. For all our racing applications the Strange Pro nodular iron case is even beefier and features a patented system of lubrication channels to ensure long life under the most extreme conditions.

12 This center section features a spool — This center section features a spool. It eliminates any sort of differential for the ultimate in traction, however they should never be considered for street-driven cars.

Are All Ford 9-inch Rear Ends The Same?

Another common question about 9 inch rear ends is what spline axle to use. That is a surprisingly complicated question as it depends on the engine’s horsepower, weight of the car, tire size, gear ratio, suspension design, and how the car will be used. While it’s hard to make specific recommendations, consider this:

28-spline axle: base
31-spline: 35 percent stronger
33-spline: 60 percent stronger
35-spline: 77 percent stronger
40-spline: 198 percent stronger

Along with the variety of axles, there are several types of 9 inch Ford Axle Bearings available from Strange Engineering.

13 Some of the features that make a Ford 9 inch are the large front pinion bearings — Some of the features that make a Ford 9-inch are the large front pinion bearings and a third bearing that supports the end of the pinion to reduce deflection under large loads.

Ford 9 Inch Sealed Axle Bearings

As they explain it, “Sealed ball bearings and sealed double-row ball bearings are typically sold as a sealed bearing. This means that the bearing is not lubricated using a rear end lube and doesn’t require a conventional seal in the axle housing. The outer diameter of this type of axle bearing is secured to the housing in such a way as to perform a seal. The inner race of the bearing is press-fit to seal the axle shaft and the sealed axle bearing is packed with grease; making it self-lubricating.”

14 The two front pinion bearings fit in a removable support that attaches to the differential case — The two front pinion bearings fit in a removable support that attaches to the differential case. The additional pinion support bearing fits into this boss in the housing (arrow).

Ford 9 Inch Tapered Axle Bearing

Tapered axle bearings are available and Strange advises they are better for continuous use in applications where side loading is frequently encountered. Circle track and street/strip vehicles often use tapered axle bearings for their ability to resist the sheer force associated with continuous and intense cornering. Tapered axle bearings sold by Strange are manufactured by Timken and may require alternate lock rings and seals (other than stock). Regardless of the type of bearing used, they should be pressed on at room temperature. Heating can reduce holding power and ruin the seal surface, so forget what you might see on some TV shows. Also, tack welding the retaining rings can alter the axle’s heat treating and cause cracking.

15 Many rear ends such as the Dana GM 10 12 bolts 8 8 Ford and others mount the differential assembly directly into the axle housing — Many rear ends, such as the Dana, GM 10/12 bolts, 8.8 Ford, and others, mount the differential assembly directly into the axle housing; making gear changes is a complicated process.

What Gear Ratio Do I Need?

When selecting rear end components one of the most complicated decisions is what gear ratio to choose. There is a long list of considerations involved–desired rpm at a given vehicle speed, tire size, transmission final drive ratio—all of which can be plugged into one of the calculators found online. In our case we wanted a gear ratio that would allow the engine to run at 1,800 to 2,200 at highway speeds in overdrive, so for our combination we chose a 4.11:1 gears.

16 Salisbury style rear ends are instantly recognizable by their removable rear cover — Salisbury-style rear ends are instantly recognizable by their removable rear cover.

Despite the fact Ford hasn’t produced a 9 inch in over 30 years, Strange Engineering can supply every part necessary to build a completely new rear end with components that are better than the originals. Custom axle housings, super-tough axle shafts, differentials, and gear sets in a variety of ratios are all available—Strange, but true. MR