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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.
If the axle tag is missing, as most are, you need to remove the third member in order to know what ratio you have and if it has a limited-slip or not.
This custom axle tag was obviously hand cut as evidenced by the sharp tin-snipped-looking ends and includes DSO, for Dealer Special Order. Notice that the date code is C9UA with the ring gear size of 8.7. This was an 8.75-inch ring gear in a 9-inch housing. These were used sporadically until about 1969. You can install a true 9-inch gear in these housings as well.
Don’t be surprised if the tag you have does not exactly match this. There are also unique domestic special-option tags, which don’t follow the format.
Date Code
The date code is stamped on the tag. The first number is the last digit of the model year of the decade, the next digit is a letter and represents the month (A is for January, B is for February, and so on), and the last two digits represent the day of the month. This format is also applicable to casting date codes.
The date code on this axle tag is 7AC, and it corresponds to the 7th year, 1st month (January), and 3rd day. The WFE-V2 helps us find the decade of the 1970s. This axle tag belongs to a 1977 F-150.
A metal axle tag is typically located on the driver-side front of the third member. It is underneath one of the nuts that holds the third member in place. In most cases, the tag has been lost. On this particular third member, the original copper washers that were under the factory fasteners have been discarded as well.
Some tags reference the week of the month instead of the exact day and just use a letter (A to E) for first through the fifth week of the month.
Case Type
You are looking for a nodular iron case, with its telltale “N” cast into the front or inside wall. These third members are the strongest of the production cases and the most desired. Aftermarket vendors are actually re-casting these cases in very strong nodular iron, so more often than not it is easiest just to purchase a new case.
But if you’re looking for an original, they can be spotted quite readily even without an axle identification tag. The N cases had two vertical ribs, three horizontal ribs, and a machined-in fill plug. There are four versions of the nodular iron third members that were produced from the factory. These first became available