High-Performance Differentials, Axles, and Drivelines. Joseph Palazzolo
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shop. Of course, it is nice to do it yourself; that way you are more familiar with the hardware and can easily diagnose and repair any issues that may arise.
After I review the exotic and cool-looking Quick Change axle, I will cover the more mainstream axle housings in alphabetical order, by the original equipment manufacturer (OEM) or by the axle supplier (e.g., Dana Corporation), when similar variants of that axle are used by multiple OEMs. The discussion will progress from small to large axles that are typically found under most muscle cars. (I refer to the axle housing by the ring-gear outside diameter in inches.)
Quick Change
One axle should be treated separately from the rest, as it is unique, but still common among hot-rodders and racers. No axle book would be complete without discussing the Quick Change axle. Halibrand Engineering was one of the original manufacturers of this style axle, and still makes them today. Winters Performance is a great source for everything from complete axles to any of the service parts. This axle design dates back to the 1940s and is common on many circle track race cars, such as Midget, Sprint or Champ, and especially Ford Roadsters from the 1930s and 1940s.
This GM 12-bolt axle has been cut and welded way too much, but it has the period-correct replacement brake lines. The four-link brackets were welded on and then subsequently cut off. The owner of this axle actually pulled it out and replaced it with a weaker 10-bolt axle because he was having so many problems with gear noise. Chances are pretty good that the main case is so badly distorted that offset and perpendicularity are off. If you run across a 12-bolt in similar condition, stay away from it.
This exploded view of an axle shows the power flow through the assembly. The input from the driveshaft is shown on the upper right portion. Then it drives a shaft that goes to spur gears in the back of the housing, then eventually transfers torque to the spiral bevel gears. (Winters Performance)
A typical quick-change axle (shown from the back) has the oval cover positioned vertically in the rear, so it’s easy to identify. These axles definitely are pleasing to see under a street rod. This one has a finned cover with gold fasteners. (Winters Performance)
On this common quick-change axle with the cover removed, you can see the spur gears that allow for the ratio change. Also, notice that the smaller gear is on the bottom, which yields a high gear ratio of 1.66:1. (Winters Performance)
These axles use a spiral bevel gear set (more on it in Chapter 6) and spur gears to obtain the overall gear ratio. The main advantage is that the spur gear portion of the axle can be removed quickly, typically from the rear of the axle, to change the ratio. This allows you to tweak your axle ratios for different tracks. It is also handy if you want to have a certain ratio for street use and change the ratio quickly for track events. As of this printing, there are more than 65 different ratios to choose from.
This is similar to the previous photo, except the gears have been changed in position. This yields a lower ratio of 0.6:1. (Winters
The powerflow for the Quick Change is also unique. The input from the driveshaft is passed through the housing via a shaft to the rear lower spur gear. This lower spur gear drives torque to its mating partner that is directly above it. The driven spur gear is attached to the spiral bevel pinion gear. This pinion gear then drives the ring gear, and finally the axle shafts.
Since the Quick Change axle uses a spiral bevel and spur gear arrangement, it produces gear noise. As always, there are trade-offs. In racing applications, gear noise is not an issue. Keep in mind that there is also an efficiency benefit of spiral bevel and spur gears over hypoid and helical gears. Since the spiral and spur gears are more efficient, less heat is generated in the axle and less torque lost from that heat generation.
Even for the street rod culture, this gear noise may not be desired. For normal muscle cars that see daily driving, you probably don’t want to deal with the noise from these units. Just as with many choices covered in this book, it is a matter of personal preference. There are helical gears available for these axles to tone down some of the noise signature, but in the end, you should expect these axles to be noisy.
The method to change the ratio is simple. You just need to remove the rear cover and replace the spur gears with your new ratio gears. There is no special shimming or preloading. Of course, you have to deal with draining and filling the gear oil. This is minor compared to what is required for a typical axle gear swap.
AMC 20 Axle
The American Motors Corporation model 20 was found in V-8–powered Javelins, Pacers and even the Matador from the late 1960s to the late 1970s. Versions of this axle were also used on select Jeep vehicles into the mid 1980s. It has an 8.875-inch ring gear, and the gearing portions of the axle are reasonably strong. The axle tubes are a little on the small and thin side along with the two-piece axle shafts. One-piece shafts and full-float kits are available in the aftermarket to address the marginally durable factory shaft and hub arrangement. The small diameter, light-duty tubes are, more often than not, bent and distorted. To make matters worse, under high-torque applications, the tubes quite often rotate within the cast center housing.
The rear cover gasket for the AMC 20 axle is round with 12 cover bolts. This is one of the few gaskets that is cork. Cork gaskets tend to deteriorate over time and will often leak.
Chrysler 8¾-inch Axle
Besides the Dana 60 (see below), the main Chrysler axle that you may encounter in muscle cars is the 8¾ inch. These axles can be found in the following platforms (some models are listed for reference):
A typical banjo-style axle—the only banjo that Chrysler made—was first introduced in 1957 on vans, trucks, and high-performance cars. This axle went out of production in 1974. There are some variations out there, but be very careful, as there are some of the smaller 8¼-inch axles built from 1957 to 1964 that can be identified by the 1828448 casting number. These would have been installed in 6-cylinder cars. There are even some “741” axles produced in 1963 to 1964 that have the smaller 8¼-inch ring gear. We want the 8¾-inch axle.
You can identify the three variants of this axle by either casting numbers or the pinion stem diameter. The 741 was used on lighter cars and has a pinion diameter of 1⅜ inch. From 1957 to 1964, the casting number was 1820657, and from 1964 to 1972, it was 2070741 (notice the last three digits of the casting number). The 741 typically has a large “X” cast on the left side above the fill plug.
The Chrysler 8¾-inch axle-housing gasket flange has 10 holes and a round shape. There are a few different versions of the gasket with different notches on the inside diameter. This gasket fits all of them. (Randall Shafer/Joe Palazzolo)
The 742 was used on large or high-powered cars, and has a pinion stem diameter of 1¾ inch. The casting numbers were 1634985 from 1957 to 1964, and 2070742 from 1961 to 1969. The 742 may have a large