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.
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 these axles have very few parts interchangeable with the 9-inch.
The 9⅜-inch axle can typically be found on 1961 to about 1972 larger cars, such as the Lincolns and LTDs, and the high-powered pickup trucks from about the mid 1960s to the mid 1970s. These axles had large carrier bearings with an outside diameter of 3.265 inches. Some of these use the unique 3.250-inch outside-diameter wheel end bearings as compared to the typical large bearings, which are 3.150 inches. These axles are still not as strong as nodular iron 9-inch axles. There is no aftermarket support for these axles, so there’s no reason to buy one for a high-performance or racing application. Therefore, the only reason to use one of these is for a concours-type restoration for originality.
There are no performance upgrades so you are stuck with used parts or, if you are lucky, new old stock. As a result, I highly recommend that you avoid this axle for any performance application buildups. You will be better off in the long run.
But there is one good side to this axle; you can swap the third member with a 9-inch unit and retain the rest. So if your vehicle already has this housing and you want to upgrade the gears and differential, you have choices. You can even put the larger 9⅜-inch third member in a 9-inch housing but you need to machine some additional ring gear clearance in the axle housing. The bolt pattern for the third member is common between the two axle housings. I am not sure why you would ever want to do that but it is physically possible.
I could not review the history and identification of these axles without talking about the Ford part numbering system. The Ford part numbering system is well organized and structured as compared to some other original equipment manufacturers’ systems that just utilize the “next number in sequence” philosophy.
Ford part numbers typically have three components: the prefix (three or four alpha-numeric characters) then a dash, followed by the basic number, another dash, and then the suffix or revision version. In some situations, there is even a pre-prefix with a single or double digit to help provide further clarification of the part’s history.
Let’s look at the part number for the nodular third member that I referenced earlier as an example:
C4AW-4025-B
The first character is for the decade and the second digit is the year of that specific decade. The second digit is also for the year of introduction or year of last revision.
All of the above holds true until the year 1999. It appears with the Y2K scare and other factors, Ford decided to revamp its date code strategy. Parts released in 1999 and forward follow this format:
The third character is for the vehicle model code. For example, R3 is Mustang, H7 is F-Series Short Cab, L3 is F150/250. So a 2011 Mustang axle is part number BR3W-4001-KH.
Ford Part Number First Character
(8.8- & 9-Inch Axle ID)
| First Character | Decade |
| A | 1940 (often omitted) |
| B | 1950 |
| C | 1960 |
| D | 1970 |
| E | 1980 |
| F | 1990 |
