Ford Differentials. Joseph Palazzolo

F-150/ Expedition platform. Four-wheel-drive vehicles use a unique reverse-rotation-style hypoid gear set, and the pinion is set above center on the front axle. With the pinion in that location, a special provision is required to supply adequate lubrication to the pinion bearings.

      The production axle is an independent-style axle arrangement with halfshafts to the drive the wheels. The stub shafts that come out of the aluminum axle housing are a 28-tooth spline shaft. These 28-tooth side gears in the differential are unique as they have a snap-ring groove machined in them to retain the stub shafts and do not rely on C-washer retention as does the rear axle version. The main reason for the pinion to be above center is to reduce the angles on the front propshaft universal joints.

      This special 8.8-inch, reverse-cut-style gear set is also used on the Currie high-pinion, 9-inch axles. They have unique cast gear cases that accommodate the smaller 8.8-inch offset but still retain the 9-inch-style differential and bearing support structure for the ring gear. The pinion is not the traditional cartridge style as on the common 9-inch; it actually operates similar to the 8.8-inch-style gear set.

       Straddle-Mount Pinion Bearing

      Another difference between the 8.8-inch arrangement and the traditional 9-inch is that the 8.8-inch gears do not have the trunnion on the pinion head, so the high-pinion 9-inch axles do not have the straddle-mounted pinion bearing arrangement. Therefore, the high-pinion, 9-inch axle used for off-road and lifted truck applications is more of a hybrid combination of the 9-inch structure with 8.8-inch gears. This axle assembly usually drives 35-inch tires and transmits up to 400 hp. The hybrid third member can also be used in the rear axle application, again limiting power and tire size. The high-pinion third members have some unique features added to the gear case in order to catch and direct oil to the bearings.

       From left to right are: 10-inch, 9-inch, and GM G-Body 7.5-inch 10-bolt.

      It is possible to use a traditional 9-inch rear axle third member and just flip it over to drive the front wheels. But in this configuration, the hypoid gears are typically driving on the coast side of the gears, which are not as strong, with a typical reduction in load-carrying capacity of about 20 to 30 percent along with poor oil flow.

       9-inch Gear Arrangement

      Hi9 has developed an actual 9-inch gear arrangement, also called TrueHi9. Again, the pinion located above center helps with driveshaft angles and ground clearance, but it requires unique oil porting and baffling in order to catch and feed oil to the pinion bearings. This arrangement uses a unique 9-inch-style gear set, produced by Richmond Gear, that has the reverse-cut method to drive on the drive side of the gear tooth faces.

      Hi9 also offers the MegaHi9. It uses gears that are made out of the stronger SAE9310 material (as compared to SAE 8620), along with a 35-tooth splined pinion arrangement. The material change makes the gears approximately 15 percent stronger.

      The TrueHi9 design has a distinctive thrust button feature to help combat ring gear separating forces along with a unique reinforced gear case in the pinion pocket bearing area. This thrust button is just a hardened, threaded, adjustable support that is very close to the ring gear back face. When the ring gear deflects from high-torque loads, the thrust button surface actually contacts a specially machined surface on the back face of the ring gear and resists that deflection.

       Here is a comparison of pinions from a 10-inch ring and pinion set with a 35-tooth spline (left) and a stock 9-inch pinion with a 28-tooth spline (right).

      Both the TrueHi9 and the MegaHi9 units use the 9-inch-style differential, which means that all aftermarket differential options can be used in these axle housings. These axles are common upgrades for Jeep Wranglers and Ford Broncos.

       10-inch Gear Arrangement

      Recently, a unique 10-inch diameter gear set has been developed (for the off-road Trophy Truck market) that fits in the 9-inch-style gear case. It provides even more strength and torque-carrying capacity of the gears. Unique yokes are required for this arrangement because it has a larger 35-tooth spline as compared to the 28-tooth standard 9-inch pinion yoke. The axle housing requires some minor machining to obtain clearance for the larger ring gear.

      These gears are massive and special for racing applications, so the ratios are limited; even the gears are expensive. A typical cost for one of these hypoid gear sets can easily be in the $1,100 range and ratio availability is sporadic. If a specific ratio demand slows down, then the ratio is no longer produced until demand picks back up. Once the demand reaches a certain level, a run of a specific ratio is made. Typically the following have been or are available: 4.11, 4.29, 4.71, 5.00, 5.14, 5.29, 5.37, and 5.43:1. Due to the high cost associated with producing these gears availability is ever changing.

       Off-Road Upgrade

      A unique 7075 billet-aluminum pinion cartridge with a washer and retainer ring, which are installed after the pinion seal, is another common upgrade for off-road applications. This setup is typically used with the 10-inch gear. The positive retention helps to protect the seal and makes certain that the pinion seal does not pop out of the cartridge during off-road use. This setup is often used on units installed in Trophy race trucks.

      A previous version of this style of seal retention just used bolts to retain the pinion seal. That style was a little better than just the seal alone but was not ideal. The seal was still vulnerable to damage from hitting something from the top or bottom. Typically the seal then pivots on the bolts and still ends up failing. The counter-sunk retaining ring arrangement from Currie Enterprises cured all of these potential failure situations.

       This is a typical modified pinion cartridge. It has bolts with washers to hold the seal in place.

       This is a much better improvement over the bolted-style retention. It uses a counter-sunk retaining ring arrangement to better retain the seal in place.

       The 9⅜-inch third member is often mistaken as a 9-inch. The top horizontal rib curving down on the right side is the easy indicator of this imposter. This axle has no aftermarket support for parts or upgrades and is not a good candidate for any performance application. The only reason to utilize this axle would be if you are performing an all-original restoration.

9⅜-Inch

      The Ford 9⅜-inch axle is often mistaken as a 9-inch. A 9⅜-inch axle has an obvious visual difference in the third-member casting rib structure of the curved top horizontal rib. Carefully inspect the axle, so you do not buy the wrong one. There’s more than a few of these at swap meets and scrap yards that may be unknowingly misrepresented as a 9-inch, and these axles are not ideal for high-performance applications. Therefore, don’t make the mistake and assume that every Ford banjo-style axle is a 9-inch. The age-old advice applies—buyer beware—as