Ford Differentials. Joseph Palazzolo

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       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.

9-Inch

      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 are in shock and spring mounting brackets and, of course, brake hardware. It is a banjo-style axle as is the 8-inch. It was in production on many Ford cars and trucks, first appearing in 1957 and finally replaced by the Salisbury-style 8.8-inch axle in 1986, saving about 50 pounds and boasting increased efficiency.

       Here is an example of a weld repair on a factory original housing. It was required to repair a leaking weld. The repair process requires the axle to be disassembled and thoroughly cleaned. The leaking weld area is then ground down and a MIG or TIG welder is used for the repair. This is a relatively common issue on these housings.

       The straddle-mounted pinion has a roller bearing in the gear case, which provides additional support for the pinion under heavy loads. This bearing and the casting structure surrounding it need to be carefully inspected as this is a common area for cracks.

       This pinion has the tapered bearings and collapsible spacer in place. The bearing cups are in the pinion cartridge and the third bearing trunnion roller bearing is located in the gear case. This pinion shows excessive wear on the trunnion surface and should not be re-used.

       Pinion Offset

      The 9-inch’s very large pinion offset of 2.25 inches requires the pinion to be straddle mounted, which refers to the fact the pinion head has bearings that straddle it on either side. There is a machined roller bearing race on the pinion head portion that points at the differential opposite the tapered bearing races. This additional bearing and, more importantly, the necessary casting support structure is what drives the larger hypoid offset. The additional straddle mount pocket bearing provides additional support to the pinion head during high-torque events. This bearing is one of the reasons that the 9-inch has the reputation for being bulletproof.

      There are three bearings on the pinion shaft: the two traditional tapered roller bearings for the head and tail bearings with a third cylindrical roller bearing on the opposite end of the pinion head. In order to allow for the additional straddle mounted bearing and casting support structure, the hypoid offset needs to be large enough to clear the differential case. Hypoid offset is the distance between the centerline of the pinion and the centerline of the ring gear. This is 2.25 inches for the Ford 9-inch and 1.5 inches for the 8.8-inch axle.

       Identification Tag

      People have been climbing around scrap yards for years to find the best examples of the 9-inch axle. If you are fortunate enough to find an axle with the identification tag still attached, it will help solve part of the mystery of what application the axle came from and what might still be inside.

      The axle identification tag is located at about the 3 o’clock position under the third-member mounting nut when you are looking straight at the front of the axle. These tags are typically quite beat up, twisted, and rusted over the years. These tags were not that informative on the early units, so from 1957 to 1962 the tag just referenced axle ratio. In 1963, Ford included more information.

       This is a typical axle tag that has been removed from the third member. The code, 4L11, tells us that it is a 4.11:1 ratio with a limited-slip or Traction-Lok differential. It also has a 9-inch ring gear.

      The axle tag typically has two lines of numbers and letters stamped on it. The format has changed over the years but mostly follow this sequence:. The top line typically starts with a three-digit axle model code (or the prefix of the part number), followed by a dash, and then the suffix. The axle model codes are interchangeable, typically the suffixes are different for a revision change but the axle is still interchangeable with one having the previous suffix. There may be a second dash and more numbers and letters on the top row if there are specifics that are unique for interchange information. Typically with just this information, you can cross reference what you need to know about the axle. The last set of digits on the top row on the right side is the date code.

      The bottom line begins with the ratio being the first set of numbers. If an L is included, it was equipped with a limited-slip differential or Traction-Lok. The middle number is the ring gear diameter in inches, typically an 8 or 9. The last set of numbers is the vehicle plant code.

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