Building a Modern 9-Inch Rearend With Strange Engineering

By Ron Ceridono

Of all the OEM components embraced by performance enthusiasts, Ford’s 9-inch rear axle assembly has to be at the top of the list. Over the years they’ve become almost standard equipment under modified vehicles regardless of the make, and for good reason. They’re tough, a wide variety of gear ratios are available, and changing centersections, or the pumpkin as it’s often called, is easy.

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02 A direct bolt in for ’65 72 Ford F 100 pickups this Strange HD 9 inch housing comes with the spring pads in place
A direct bolt-in for ’65-72 Ford F-100 pickups, this Strange HD 9-inch housing comes with the spring pads in place and the shock mounts moved down 1.250 inches and forward 2.75 inches to improve suspension geometry for lowered vehicles.

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Not exactly new on the scene, the 9-inch was produced from 1957-86. To put things in perspective, when the last of these rearends came from the factory under a new Ford, Ronald Reagan was in the White House, the average price of a gallon of gas was $0.90, and Seinfeld was the number one TV show. Given the fact that the 9-inch has been out of production for quite some time it would seem that complete assemblies and even individual parts would be hard to come by, but that’s not so. Strange Engineering is well aware of the advantages to the 9-inch Ford rearend and can provide everything new, from individual parts to complete assemblies.

03 complete centersection can be removed from the housing and quickly swapped for another
One of the advantages to a Hotchkiss design is the complete centersection can be removed from the housing and quickly swapped for another.

The design of the Ford 9-inch is Hotchkiss style, which is to say the case containing the third member, or the ring-and-pinion assembly, 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, which makes gear installation and adjustment much more complicated.

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04 Salisbury style rearends such as the Dana GM 10 12 bolts 8 8 Ford and others mount the differential assembly directly into the axle housing
Salisbury-style rearends, 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 a complicated process.

A notable feature of the Ford 9-inch is both a big advantage as well as a slight disadvantage. To improve the tooth contact and strength, as a result, the pinion is low on the ring gear when compared to other designs. As explained by Strange, “This larger offset (hypoid distance) between ring gear and pinion gear centerlines is the key reason the Ford is inherently stronger but also why it’s slightly less efficient.” Arguably one of the reasons for the demise of the 9-inch was the effort to increase fuel economy with a more efficient, albeit somewhat weaker, rearend.

05 Salisbury style rearends are instantly recognizable by their removeable rear cover
Salisbury-style rearends are instantly recognizable by their removeable rear cover.

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Another reason for the popularity of the 9-inch is the method of retaining the axles. These rearends use 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, and rear disc brakes act as retainers to some extent).

06 This is an “open” differential
This is an “open” differential. The gears with the white marks are the side gears attached to the axles (A) and the spider gears (B) that allow the wheels to travel and different speeds when cornering.

Strange offers a variety of new 9-inch housings. For our application, a ’71 Ford pickup, we chose an HD housing (PN HF9FT65F100ME). The centersections are fabricated from 0.141-inch mild steel with internal gussets for rigidity; the stout axle tubes are mild steel, 3-inch-diameter, 0.250-inch wall thickness. Another consideration when selecting an axle housing is what axle ends are required, which is dictated by the axle bearings that will be used. There are three distinct 9-inch axle ends: the early big bearing, the late big bearing (Torino), and the late ’65-73 small bearing. We opted for the early big Ford ends (PN H1135) with tapered wheel bearings (PN A1013). These bearings offer exceptional life in street applications as they handle side loading much better than ball bearings.

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07 This is a typical clutch style limited slip unit
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 side gears (A) on the axles and spider gears (B).

When selecting rearend components, a common question is what type of differential to use. As a car, or in this case a truck, turns a corner, the outside drive wheel travels further than the inside wheel. As a result, the outside wheel has to turn faster—the differential allows this to happen. The most common type of differential is the “open” style. The problem with this design is that when one tire loses traction the differential will transfer all the power to that wheel. The proof of that is the well-known one-tire burnout. To put power to the ground from both tires there are a variety of limited slip units, aka Positraction differentials, available. We picked an Eaton Truetrac (PN N1979) for its smooth, seamless operation and durability. In addition, they don’t require special lubricant-like, clutch-style units and aren’t noisy like a locker.

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

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The heart of the rearend assembly is the centersection. Strange offers a variety of case assemblies in iron and aluminum. For our application we went with an S-series nodular iron case (PN PRF130). Stronger than the factory nodular iron case, it comes with a cast-iron Daytona pinion support (PN N1914) that uses larger bearings compared to the standard factory support and a heavy-duty 1350-series U-joint yoke (PN U2203). This case will accept a wide variety of ring-and-pinion ratios; in our case we chose a 4.11:1 gearset from U.S. Gear (PN RS07890411US).

09 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.

One of the common questions when it comes to selecting axle is spline count, which is another way to try and quantify strength. Unfortunately due to a variety of factors, including vehicle weight, horsepower, tire size, and so on, coming up with meaningful strength numbers for axles is impossible. So consider this: Base 28-spline axles are reasonably tough, but for performance applications use 31-spline axles because they are 35 percent stronger and are appropriate for most street-driven vehicles. For more serious applications when compared to 28 splines, 33 splines are 60 percent stronger, 35 splines 77 percent stronger, and 40 splines are 98 percent stronger.

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

The Ford 9-inch rearend assembly was introduced 66 years ago and has been out of production for 37 years. Yet today this mainstay for performance enthusiasts is better than ever thanks to Strange Engineering’s effort to improve them. It just goes to prove some things really do get better with age.

11 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 bearings fit into a recess housing.
12 Here the pinion gear assembly has been installed in the case
Here the pinion gear assembly has been installed in the case. A torque wrench is used to measure the preload on the bearings.
13 This is an Eaton Truetrac This helical gear style limited slip differential is quiet and smooth
This is an Eaton Truetrac. This helical-gear style, limited-slip differential is quiet and smooth in operation with durability comparable to that of the locker. They are recommended for street/strip applications.
14 Strange uses quality gearsets from Motive Gear and U S Gear The bearings are from Timken
Strange uses quality gearsets from Motive Gear and U.S. Gear. The bearings are from Timken.
15 Part of adjusting the mesh of the ring and pinion is setting pinion depth
Part of adjusting the mesh of the ring-and-pinion is setting pinion depth. That is adjusted with shims (arrow) between the pinion carrier and the differential housing.
16 Threaded adjuster nuts in the housing are used to move the ring gear toward or away from the pinion to adjust backlash
Threaded adjuster nuts in the housing are used to move the ring gear toward or away from the pinion to adjust backlash.
17 Properly setting up a ring and pinion is practically an artform—the technicians at Strange have the procedure down
Properly setting up a ring-and-pinion is practically an artform—the technicians at Strange have the procedure down. Here the pattern indicating the mesh of the ring-and-pinion is checked with marking compound.
18 Here a dial indicator is being used to determine the backlash or the play between the gears Strange recommends 0 010 to 0 012 inch
Here a dial indicator is being used to determine the backlash, or the play between the gears. Strange recommends 0.010 to 0.012 inch.
19 Once the gears are adjusted the technician’s initials date of assembly and adjustment info is engraved on the ring gear
Once the gears are adjusted, the technician’s initials, date of assembly, and adjustment info is engraved on the ring gear.
20 the Strange nodular iron case is up to 10 percent stronger than the iron used in factory rearends
At one time the factory nodular iron case was the hot setup. Today the Strange nodular iron case is up to 10 percent stronger than the iron used in factory rearends and up to 20 percent more ductile
21 There are two different length axles used in a 9 inch rear
There are two different length axles used in a 9-inch rear: the short axle is on the driver side, the long axle is on the passenger side (ours are 30-3/8 and 28-5/8 inches). Strange 31-spline alloy axles we chose are considerably stronger than the OEM shafts.
22 The bolt pattern for our application is 5 on 5 5 bolt circle
The bolt pattern for our application is 5-on-5.5 bolt circle. The large hole in the axle flanges provide access to the bearing retainer fastener and rather than press-in wheel studs, the flanges are threaded and high strength capscrews are used.
23 Bearings and lock collars should always be pressed on the axles at room temperature
Bearings and lock collars should always be pressed on the axles at room temperature. Pre-heating is not recommended as collars can lose holding power, and never “tack weld” collars as damage to the axle’s heat treat can occur.
24 Our Timken tapered wheel bearings (PN 1013) require inner seals (PN A1030)
Our Timken tapered wheel bearings (PN 1013) require inner seals (PN A1030).
25 The U shaped retainers hold the axles in the housing
The U-shaped retainers hold the axles in the housing. It fits against the wheel bearing so in the unlikely occurrence an axle brakes, the wheel won’t come off the car.
26 Salisbury rearends use a C clips that fit into a groove in the end of the axle as a means of retention
By comparison, many more modern Salisbury rearends use a C-clips that fit into a groove in the end of the axle as a means of retention.
27 A vent in the housing (PN H1112B) prevents buildup of internal pressure and ensures sealing integrity
A vent in the housing (PN H1112B) prevents buildup of internal pressure and ensures sealing integrity. To facilitate fluid changes or housing was equipped with fill and drain plugs.

Source:

Strange Engineering
(800) 646-6718
strangeengineering.net

Click on this issue’s cover to see the enhanced digital version of Building a Modern 9-Inch Rearend With Strange Engineering.ctp april 2024

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