increased efficiency. The 9-inch axle has a very large pinion offset of 2.25 inches, which allows the pinion to be straddle mounted. There are three bearings on the pinion shaft. In order to allow for the additional straddle-mounted bearing, the hypoid offset needs to be large enough to clear the differential case.
Many people have been scouring scrap yards for years to find the best examples of the 9-inch axle. Most are looking for the highly sought-after nodular iron case, with its telltale “N” cast into the front or inside wall. The N cases had two vertical ribs and three horizontal ribs along with a machined-in fill plug. Of course, if your pocket book allows, you can easily purchase an aftermarket iron case that is stronger than any factory case. If you are looking for the N case or think that you have found one, make certain to look closely. The WAR, WAA, and WAB cases have the same ribbing as the N case, but are missing one feature; the W-series cases do not have a fill plug machined into them. As always, do your homework before you spend those hard-earned dollars. Both standard and W-series cases are made out of gray cast iron. Only the N-series cases are made out of the stronger, more desirable, nodular iron.
There are even different pinion cartridges. The Daytona-style cartridge has additional structure, and allows for a larger pinion head bearing when compared to the standard pinion cartridge.
Like most Ford axles, the 9-inch has a single hypoid ring-gear mounting distance, so unlike the Dana and GM axles, a single differential works with all ratios. Also, since this axle is used in so many circle track race cars, the 9-inch enjoys an unparalleled availability of different gear ratios.
The Ford 9-inch was the axle most commonly found in Blue Oval muscle cars and trucks, and this axle is so prolific that aftermarket companies make it for Chevy applications. The Ford 9-inch internals in the back side of the center section or third member look like this. You can see the adjuster nuts for the differential location and bearing preload.
This is the front side of the third member. The pinion cartridge is held in place by five bolts. This unit has a more common 3/8-inch-square-drive fill plug.
An aftermarket Daytona-style pinion cartridge is more reinforced than the standard cartridge and accommodates the stronger pinion head bearing. This additional reinforcement comes from the thicker section inner ribbing.
This description by no means does justice to all permutations of the ever-popular Ford 9-inch, but we’ve covered the basics. If you have one of these in your muscle car, or want to use one, research further to get all the details. This is where making a few phone calls to reputable axle builders can be helpful. Quite a few companies are making Ford 9-inch housings to fit just about any muscle car out there, including non-Ford vehicles.
General Motors Axles
The last axles to review are two of the more common GM axles. These axles account for the majority of muscle car applications. GM called its limited-slip differentials “Positraction.” This name was shortened, and the term “Posi” was coined and is now synonymous for limited-slip differentials. However, just knowing that it has a Posi does not tell us which technology is being used (see Chapter 5 for more details).
10-Bolt: The GM 10-bolt axle housings get their name from the 10 fasteners that hold the rear inspection cover in place. The rear cover identifies it as a Salisbury-style axle. Both the 10-bolt and 12-bolt require unique differentials to accommodate different ratios. There are Series 2, 3, and 4 differentials to account for unique ring gear mounting distances (more on this in Chapter 6). The 10-bolt and 12-bolt axle housing share a common lubrication flow strategy to the differential bearings. Oil is collected from the rotating ring gear and channeled toward the differential bearings and then down the axle tubes to the wheel end bearings. A shelf that is stamped into the rear cover accomplishes this.
The 10-bolt axles came with two different-size ring gears for passenger car applications: an 8.125-inch (commonly referred to as 8.2-inch) and an 8.5-inch. Basically, most applications in the mid 1960s to early 1970s with 10-bolt housings had an 8.2-inch ring gear. By the early 1970s, most of the 10-bolt housings in Chevelles, Novas, Camaros, Monte Carlos, etc., had the larger 8.5-inch ring gears.
These axles replaced the 8.2-inch banjo style that was available prior to this. For any high-powered application, get the larger ring gear that came with the 12-bolt housings. If you have a high-performance small-block or big-block in your muscle car, you will want to upgrade to the stronger 12-bolt. There are plenty of 10-bolt muscle cars that have great results, but the 12-bolt is a far stronger unit.
12-Bolt: Just like the 10-bolt, the 12-bolt gets its name from the number of cover bolts that hold the rear inspection cover in place. It is a Salisbury-style housing with a cast center section and press-in tubes. The rear cover of this axle has a much more pronounced lube shelf to distribute oil from the ring gear to the differential bearings.
The axle tubes are held in place by the tight interference fit and plug-welded joint to the center section, as are other examples of this style axle housing. Unfortunately, the 12-bolts have the tendency to seep gear oil out of the plug welds and the tube-to-case interfaces.
Many misguided mechanics have attempted to stop 12-bolt axle leaks only to find that the true root cause is the welds or press fit. From the factory, the plug welds had pin holes and were just plain sloppy. To repair this correctly seems easy—grind out the old welds, clean up the interface, and weld it correctly. This is not as simple as it might seem, since you are welding two dissimilar ferrous materials–cast iron to mild steel. Take your time and grind out the entire old weld without grinding into the tube, then arc weld the two back together. Make certain that you use the correct filler material. Most folks recommend a 304 stainless rod for this weld. If you are not a good welder, leave this repair to a professional.
On the left side of the aftermarket chrome cover for the GM 10-bolt, a curved bulge acts as a shelf that collects and diverts oil to the differential bearings. There is a similar feature on the right but it is not as pronounced.
Also, while you’re at it, have the tubes welded to the cast center section. This not only stops those leaks, but also adds more support. Make certain to have a shop experienced with axles perform this work so that the housing does not get distorted from the heat of the welding process.
A common chrome aftermarket GM 12-bolt cover features the distinctive oil shelf across the top portion of the cover. This makes these axles very easy to identify because it is the only axle with this style cover. The chrome plating adds a nice touch if you want a show part but does not really add any additional strength.
Unlike stock units, aftermarket structural rear covers incorporate fill and drain plugs. Another great benefit is the differential bearing cap preload bolts. These are at the 3 and 9 o’clock positions of the cover.
These housings are still highly sought
