Ford Differentials. Joseph Palazzolo
to improve reliability, prevent component damage, etc.
Documentation Required—
Illustrates a point in the rebuild where the reader should write down a particular measurement, size, part number, etc. for later reference or photograph a part, area or system of the vehicle for future reference.
Tech Tip—
Tech Tips provide brief coverage of important subject matter that doesn’t naturally fall into the text or step-by-step procedures of a chapter. Tech Tips contain valuable hints, important info, or outstanding products that professionals have discovered after years of work. These will add to your understanding of the process, and help you get the most power, economy, and reliability from your engine.
AXLE HISTORY AND IDENTIFICATION
Before you embark on the rebuild and start scouring local scrap yards and online websites, it will be helpful to understand what to look for and what to avoid in these axles.
First, you need to understand some fundamentals of the Ford axles to help guide your quest. I have spent countless hours over the years collecting, reading, and studying old shop manuals, supplier reference documents, SAE papers, and even vehicle manufacturer reports and notes. Some of this information is very important as it is becoming more and more difficult to find documentation on axles and differentials that were built more than 40 years ago. I have also spent many hours in scrap yards and visiting and interviewing the experts in this field along with years of building numerous axles myself. In this chapter I share a summary of decades of work on these axles.
As part of my research, I went to renowned Currie Enterprises and gathered one of just about every different 9-inch axle variant. I put together a whole collection of housings and third members. This is a sampling of some of them. Here is a quick visual summary: The 1957 housings do not have dimples on them and have an oil drain plug. The 1958s and 1959s have two dimples on either side of the housing and some have drain plugs. The 1960s to 1967s still retain the dimples but also have an oil level plug in the back cover.
The Ford 8.8-inch axle has a cast center section and the internal components are installed from the rear. There is typically a stamped steel cover that must be removed to gain access to the internals.
This chart serves as a general guideline for axle flange-to-flange width based on different models and production years. I have included the narrow 8-inch axles for reference. The narrowest production 9-inch axle is 56.375 inches. If measuring the drum-to-drum distance, add .200-inch to these values.
An assortment of Ford 9-inch axle stampings is welded together to create the housing. The cast-iron third member is installed from the front of the housing. The third member supports all of the gears and bearings, which allows for easier gear ratio swaps if you have multiple third members.
There is a very fundamental difference between Ford’s 8.8- and 9-inch axles. I reference these axles based on the ring gear nominal outside diameter in inches. The fundamental difference between these axles is how the gears are supported and which end of the axle housing (front or back) that they are assembled from.
It may seem strange to begin by comparing the 8-inch to the 9-inch axle, but there is an important distinction to make since the 8-inch axle is weaker than the 9-inch. Many people are not aware that Ford made a smaller banjo axle and confuse the 8-inch for a 9-inch.
The 8-inch was introduced in 1962 and is found in many lower performance Fairlanes, Mustangs, Falcons, Comets, Cougars, and Pintos, just to name a few of the applications.
There’s an easy way to tell an 8-inch apart from a 9-inch. All of the 8-inch case nuts can be accessed with a socket. In contrast, on the 9-inch, two nuts on the bottom at about the 6 and 7 o’clock positions cannot be accessed with a socket and require a wrench.
Both axles share a common design and are often referred to as banjo style or third-member style. The smaller 8-inch just cannot handle the abuse as its bigger brother can. The 8-inch also was only available with 28-tooth axle shafts. Unless you are building a Pinto or straight six-cylinder vehicle, you want to avoid it.
The smaller 8-inch third member is on the right. Note the two lower bolts at 6 and 7 o’clock have straight access with a socket. The 9-inch third-member bottom fasteners, on the left, can only be accessed with a wrench. This is an easy way to identify the third members, so you’re sure to buy the correct axle.
Both 8- and 9-inch axle housings are made from a series of stampings that are fixtured and welded together. This complex fixturing and welding process, coupled with fuel economy concerns, is what eventually led to their production demise. These axles had a stout 2.25-inch ring gear offset as compared to a 1.5-inch ring gear offset of the later 8.8-inch axle.
The larger offset is better for strength and noise but worse for sliding and efficiency. While the larger offset makes the ring and pinion gears stronger, the additional sliding of the gear teeth creates more heat in the axle. Therefore, these axle assemblies require better quality oil and good underbody airflow to keep the unit cool. With their high-volume production, the two biggest quality problems were leaks from poor welds and poor alignment of the housings. So don’t be surprised that most of these axles leak from the welds. Careful aftermarket shops and their stringent attention to repairing these housings are able to correct many of these issues.
The Ford 9-inch has a reputation as a durable axle that can transmit enormous torque, and this is rightfully deserved. Some even consider these axles to be bulletproof. This is by far the most common axle used by restorers, hot rodders, customizers, and racers. It has enjoyed a long production history with many variants. There is a huge aftermarket support for this axle design. Many companies, such as Currie Enterprises, Mark Williams Enterprises, Moser Engineering, and Strange Engineering, reproduce this design today. It is still used in NASCAR racing as well.
Because of the long production history of this axle, many variants are available. Most of the time, the differences