The time has come that your factory axle is in need of repair. You have decided that the 30-year-old housing is worn and it may need to be replaced or upgraded. Now it is time to start scouring friends’ garages and the local scrap yards to find your axle treasure. Before you begin the quest, you need to understand what to look for in your new axle housing.
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First I will cover some generic types and axle structures. I have spent many hours collecting, reading, and studying old shop manuals, supplier reference documents, SAE papers, and vehicle manufacturer reports and notes. Some of this information is very important because is it becoming more and more difficult to find documentation on axles and differentials that were first introduced more than 40 years ago.
There are two main categories or styles of axle housings available: beam style axles and independent-carrier style axles. The main difference is that the beam-axle housing has axle tubes as part of the axle housing; the independent carrier-style axle is just the axle center section that houses the hypoid gears and differential.
Beam axles use straight axle shafts, and the entire axle housing moves during suspension travel events. The beam axle also reacts to suspension loads through the shocks and springs. When the suspension loads are transferred to the axle housing, it implies that the vehicle loads are also transferred to the housing. The more heavily loaded the vehicle, the more loads the axle must resist. This is more important on light duty pickup trucks, but does relate to muscle cars, as you may stuff five passengers in the car and go for a zip around the block to show off your new limited-slip differential. The beam axle housing must also react to the braking loads. The brake drums and wheel cylinders (or calipers and rotors) are mounted to the wheel ends of the tubes. Some axle housings even locate the vent fitting in the top of one of the axle tubes.
Banjo and Salisbury Axles
There are two main types of beam axle housings banjo and Salisbury and both of these styles are still in production today. Toyota and Mitsubishi are a couple of the vehicle manufacturers that use banjo axles today while most other manufacturers have switched to Salisbury. All Salisbury beam axles have a cast center section.
There is an interesting history behind the term Salisbury axle. In 1901, the Salisbury Wheel Company was founded in Jamestown, New York. The founder, C.W. Salisbury, patented an automobile wheel. He was a key maker and repaired umbrellas by trade. He paired up with Scott Penfield and E.D. Sherman to start his manufacturing process. Their first customer was the producer of the Thomas Flyer Auto-mobile, E.R. Thomas Company.
In 1905, the Salisbury Wheel Company started producing front axles and, in 1907, began producing rear axles. Around the same time frame, Clarence Spicer started the Spicer Company in Plainfield, New Jersey, not too far from Salisbury, and specialized in universal joints. Charles Dana, an attorney and businessman, invested in Spicer around 1914. In 1919, Spicer Manufacturing Company acquired the Salisbury Axle Company. Spicer Manufacturing is now called Dana Holdings Corporation, but the Salisbury name is still around.
This style of axle became very popular and, with the advent of World War II, many Jeeps were outfitted with this style axle. This axle style was well known in England from the Jeeps during World War II, and the English still refer to this axle as a Salisbury. Just keep in mind that these axles have nothing to do with Salisbury, England. Even today, we still associate the term Salisbury axle with an axle that has a cast center section, rear cover, and pressed-in tubes.
The banjo-style axle has a removable center section. This cast center section is removed from the front of the axle and supports all of the gears and bearings. Many small steel pieces are welded together to form the banjo-style axle housing. Toyota still uses this style axle housing on its truck platforms today. The most well known banjo axle is the Ford 9-inch, which will be covered later. The banjo axle, with its studs sticking out, resembles its namesake musical instrument, with the drum and tuning keys sticking out.
Probably, the most significant weakness and most common failure point of the banjo axle is at the differential bearing supports, which are cantilevered out from the cast structure. The design has two arms reaching from the main casting, like diving boards, that are trying to support the hypoid-ring-gear separating loads. Under heavy torque loads, these bearing supports can, and frequently do, break in stock form. If you are going to send tremendous amounts of torque through a banjo-style axle, it is crucial that you use a readily available aftermarket center section, or a factory nodulariron unit that is stronger than the common, stock-gray-iron center section found on most passenger car applications.
Independent Carrier Axles
Independent carrier axles (ICAs) utilize halfshafts instead of axle shafts to transfer torque to the wheel-end hubs. ICAs, therefore, do not react to suspension loads. The axle-carrier movement is typically independent of suspension movement. These style axles are more commonly found on luxury vehicles and ultra-high-performance cars, such as Ferraris. Certain domestic performance vehicles have also utilized ICAs. The purpose built, limited-production 1999 Cobra R Mustangs debuted with an ICA, and all Cobra Mustangs from 1999 to 2004 were factory built with ICAs.
The latest Dodge Vipers and Corvettes use ICA rear axles, and the reinvented 2010 Camaro has an ICA. Many folks have grafted ICAs from other applications into their muscle cars. I need to caution you on performing such a modification, as the ICA load-bearing frame members, mounts, and dampers are originally tuned for the original chassis stiffness and compliance.
When you rip that subframe out of a Jaguar or Cadillac CTS and make it fit under your Nova, chances are it will not perform the same as the original donor car because the entire rear subframe module is tuned for a specific chassis arrangement, wheelbase, and weight transfer characteristic. Keep in mind that there are companies that specialize in conversion kits for such swaps. These are expensive swaps and do not always garner the expected benefits. A custom fabricated ICA can be a great talking point while at a car show. In some racing circles, an independent suspension may yield significant advantages, especially when there are abrupt changes in the track surface.
The current Corvette axle is unique in the fact that the pinion is located behind the ring gear when the ICA is installed in the vehicle. The pinion shaft is actually hollow and driven from an internal spline in the head of the pinion. Conventional pinions are driven from an external spline at the tail end of the pinion. The Corvette pinion is also very short with a small distance between the pinion bearings. Using large pinion bearings makes this arrangement possible and provides adequate support for the pinion gear.
Rear Cover and Side Cover Housings
There is a subset of ICAs defined by the cover style or split line of the axle housing. Most ICAs have the axle cover split line like the Viper axle, which is parallel to and just behind the output shafts. There are some that utilize a side cover design like the Corvette. There are many schools of thought regarding which design is better. But one issue that all agree on is that most side-cover designs today do not easily lend themselves to visual pattern checks and traditional gear backlash inspection. Since the side cover is required to properly support the differential bearings and subsequently the hypoid ring gear, you cannot see the pattern or check backlash unless the cover is removed. If a large access port were available, a pattern and backlash check could be performed. The other method is to assemble the unit and rotate the pinion to disburse the pattern compound, and then remove the cover to check the pattern.
ICAs have been manufactured in both cast iron and cast aluminum. The cast-aluminum units have a thermal-expansion and gear alignment issue that needs to be taken into account during the design process. Aluminum expands and shrinks at a faster rate than the steel gears inside the axle. Depending on the design layout of the gears and bearings, when the aluminum axle housing heats up from normal operation, the gears may be shifted out of their ideal mesh point. Also, the pinion bearings typically increase in preload, while the differential bearings lose preload. This can turn into gear noise at elevated temperatures in the axle.
If the axle receives a cool stream of air while driving to help maintain the temperature, this heating of the axle housing is minimized and these problems do not exist. The typical ICA is tucked up under the vehicle, and does not usually receive adequate airflow. This is important if you are going to stick one of these axles under your muscle car; the transmission will have poor airflow, but also increased torque well beyond the factory design limit. Many manufacturers are now going back to cast iron for their ICAs in order to eliminate the concerns that come with an aluminum structure.
Axle Shaft Retention Methods
Just like other items previously discussed, axle shaft retention methods fall under the axle housing category. Some folks refer to the axle housing as semi-float when they mean to say Salisbury.
Let’s clear up this confusion now. There are three main types of axle shaft retention: semi-float, three-quarter float, and full-float. The type of axle shaft retention is typically easiest to distinguish based on the bearing arrangement at the wheel end.
Semi float axle shaft retention is most commonly found on passenger cars and light duty trucks. It is arranged so that the vehicle loads react to the axle shaft. It is the simplest and most cost effective design for vehicle manufacturers. This is the traditional Cwasher style axle shaft retention.
The axle shaft has inherent end-play between the differential pin and the C washer pocket in the side gear. The endplay increases if the optional limited slip differential is a plate style, and the plates wear over time. The wheel end bearing arrangement is typically a roller bearing that rides on the axle shaft. The axle shaft itself experiences torque as well as a bending load that results from supporting the weight of the vehicle through the wheels. The path of wheel loads goes from the wheel, to the axle shaft, to the bearings, and finally into the housing.
There is another method to retain the axle shaft for semi-float retention. Here, the wheelend bearing is pressed onto the axle shaft. This style of axle shaft retention is commonly referred to as the captured bearing or Ford style. Although this is a three quarter-float, it is not commonly called this. There is a bearing retainer plate that bolts to the end of the axle housing. This bearing and retained plate are pressed and secured on the axle shaft. The Ford 9 inch style axle shaft is an example. This style of axle retention has a distinct advantage over a typical C washer arrangement, since a broken axle shaft does not necessarily allow the wheel to separate from the axle housing.
C clip eliminator kits are available in the aftermarket to convert C washer axles to this type of captured bearing retention. Keep in mind that this is still a semi-float-style axle. Since it is not a true full-float axle, but awfully close, it is referred to as three quarter float.
The full-float wheel end arrangement is such that the axle shaft only experiences torque. The wheel end bearing transfers load from the axle housing to the wheel hub. This type of axle is easy to spot by the bolts on the outside flange of the axle shaft that hold the axle shaft in place. The axle shaft drives a separate wheel hub that supports the wheel structure. These are typically found on larger axles in three quarter and oneton trucks.
And there is another method to identify this axle: If you can remove the axle shaft with the vehicle on the ground, it is a full-float axle.
To determine if your vehicle has a full float wheel end, closely examine the axle shaft to wheel hub attachment. If there is a wheel hub, you have a full-float axle, but the presence of the wheel hub may not be obvious. In order to visually tell, look for a series of bolts arranged on a smaller diameter than the lug studs that attach the axle flange to the hub.
The axle beam portion or tubes can be a series of stamped-steel pieces that are all welded together like the banjo style axle, or they can be actual round-shaped tubing. The preferred tubing for this is called Drawn Over Mandrel (DOM) tubing. Most Salisbury-style axles use DOM tubing for the axle tubes. This style of tubing is also commonly used for driveshaft production. DOM tubing has great strength and consistent wall thickness properties that make it ideal for axle tubes.
There are a few different methods to manufacture tubing like DOM and seamless. The tubes actually start out as solid bar stock and are then pierced. Subsequently, the steel is heated, drawn, and formed into the correct shape. Then a mandrel or fixture is positioned inside the tube while the outside of the tube is pulled through a die. This basically sizes the inside and outside diameter of the tubing and thus the name drawn over mandrel. The diameter and wall thickness of the axle tubes is important in the overall axle strength. Some axles have very small diameter tubes and are therefore more prone to bending.
In most enthusiast circles, nothing provokes more debate than the topic of the best differential. When it comes to Chevy versus Ford versus Mopar differentials, everyone has an opinion on which performs better. The same heated conversations begin when discussing Ford 9-inch versus Dana 60 versus GM 12-bolt. Everyone has an opinion. Some opinions are based on valid information. However, some opinions are based on outdated information, not unlike the camel-hump small-block Chevy cylinder heads that were all the rage years ago. Now, with all of the aftermarket support available, unless you want to build a numbers-matching or nostalgic engine, aftermarket aluminum heads would deliver far better performance than the stock units.
Independent of any of this information, common sense should always prevail. As these axles, and the cars they came out of, become older and scarcer, more and more axles show up on the market that have been modified incorrectly, bent, and generally abused. Unless the housing is extraordinarily rare, a used part should be avoided. Alternatively, if you are planning to install new tubes and refurbish everything else that is involved, and then go ahead and buy an axle that has been excessively welded on or shows signs of damage,just be sure that you pay appropriately.
New aftermarket complete axles may not be as expensive as you might think. Keep in mind that by the time you purchase a new gear set, differential, bearings, seals, axle shafts, and brake hardware, you may be only a few dollars short of a rebuilt axle from a reputable 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.)
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.
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.
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 tradeoffs. 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 pre-loading. 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.
Chrysler 83⁄4-inch Axle
Besides the Dana 60 (see below), the main Chrysler axle that you may encounter in muscle cars is the 83⁄4 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 81⁄4 -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 81⁄4 -inch ring gear. We want the 83⁄4-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 13⁄8 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 742 was used on large or high powered cars, and has a pinion stem diameter of 13⁄4 inch. The casting numbers were 1634985 from 1957 to 1964, and 2070742 from 1961 to 1969. The 742 may have a large number 2 cast on the left side.
The 489, the last variant, was phased in to replace the 742. It has a tapered pinion stem diameter of 17⁄/8 inches. The casting numbers from 1969 to 1974 were 2881488 and 2881489. The 489 has a large number 9 cast on the left side. There were some 13⁄4 inch pinions installed in the 489 casting during this time period, so you ought to inspect the axle carefully to confirm which one you’re looking at.
So which one do you want? Supposedly, the larger the pinion stem, the stronger the axle. The axle center sections are interchangeable, so if you have the weaker 741, you can easily upgrade to a 742 or 489 without too much trouble. As with any upgrade, take the time to do your homework.
Also, take note that most Chrysler axles have left hand or reverse threaded ring gear bolts. Left-hand threaded bolts self tighten when the joint is loaded, which counteracts any tendency to loosen. These are ideal when the bolt is on the center of rotation like the bolt that holds your lawn mower blade to the crankshaft. However, ring gears and wheel studs are not lined up on the center of rotation, and therefore do not benefit from this opposite hand thread. For some reason, Chrysler used this on their ring gears and driver’s-side lug nuts in the 1960s and 1970s. General Motors used the same design philosophy for some of their axles.
Dana 60 Axle
Just like many of the manufacturers in this text, Dana has many different size axles and combinations available. I am going to concentrate on the Dana 60 (D60). It has a large 93⁄4-inch ring gear and was used in many different Mopar truck applications, passenger cars with 440, 4-speed combinations, and the legendary 426 Hemi cars. This axle is far superior in strength and efficiency to the Ford 9-inch. For some reason, Chrysler was the only OEM to use the indestructible D60 in their muscle cars.
Typically, you can build a custom D60 for one-third less than the cost of a Ford 9-inch. Unfortunately, you cannot get a D60 for certain applications one being the Ford Fox platform. However, if the ring gear in your muscle car is less than 9 inches and it cannot handle the torque input, you should opt for the Dana 60. Many sub-9-inch axles cannot hold up when horsepower rises to 800 or 1,000 or more.
The D60 hypoid offset is half that of the Ford 9-inch. Therefore, less torque is lost and less heat is generated in the D60. There are many aftermarket companies that make their own version of the D60, such as Strange and Teraflex. Most of the D60s use a solid spacer, while some use a collapsible spacer, to set the pinion bearing preload. The spacer needs to be adjusted to obtain the correct preload, and then the nut is tightened to the correct torque value.
Ford has produced its own corporate axles just like GM and Chrysler. Some of these axles are similar to the other OEMs, while others are unique, such as the Ford 9-inch. We are going to review the more common available axles.
Ford 8-inch: It may seem strange to compare the 8-inch to the 9-inch because the 8-inch axle is weaker than the 9-inch. However, an important distinction needs to be made. Many people are not aware that Ford made a smaller banjo axle and confuse the 8-inch with a 9-inch. There’s an easy way to tell them apart. An 8-inch has case nuts that can be accessed with a socket, while the 9-inch has two nuts on the bottom around 6 and 7 o’clock that cannot be accessed with a socket and require a wrench. Both axles share a common design, but the little 8-inch just cannot handle the abuse like its bigger brother can. The 8-inch also was only available with 28-tooth axle shafts.
Also, don’t confuse the later pictures in this chapter that show independent 8-inch carriers with the 8-inch banjo-style beam axle housing. Both 8-inch 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, most likely led to the production demise of these Ford axles. In 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, with their stringent attention to repairing these housings, are able to correct many of these issues.
Ford 8.8-inch: The Ford 8.8 inch is very similar to the GM 87⁄8-inch, 12-bolt axle. Some folks may tell you that the Blue Oval engineers copied the 12-bolt design when they came up with the 8.8-inch axle. While that is a matter of opinion, one should recognize that there are many similarities. It is interesting that the Ford axle uses the exact same bearings as the GM 12-bolt. The Ford version uses larger axle shafts and a different lube strategy, but otherwise they are very similar axles.
Another interesting fact is the Ford 8.8-inch axle has the same ringgear mounting distance (see Chapter 6) for all ratios. There is a ton of aftermarket support for this axle (see Chapter 4). This is your traditional semi float Salisbury axle with C washers for axle retention. There are even 8.5 inch gears installed in these housings for lower-powered vehicle applications. The typical car-sized 3-inch-diameter tubes can be a bit flimsy in higher powered applications.
The 8.5-inch and 8.8-inch gears have even been found in the independent carrier style axles. The Ford Mustang and T-Bird used this style for a few model years.
Ford 9-inch: The Ford 9-inch has a reputation as a very reliable and durable axle. It’s by far the most common axle used by restorers, hot rodders, customizers, and racers. It has enjoyed a long production history with many variants. For its time, this remains one of the best axles to use. There is a huge aftermarket support for this axle design. Many companies are reproducing this design today, such as Currie Enterprises, Mark Williams, and Strange Engineering, to name a few. It is still used in NASCAR racing today.
Offered in Ford cars and trucks from 1957 to 1986, this banjo-style 9-inch axle is the big brother to the 8-inch axle. In the mid 1980s, the Salisbury-style 8.8-inch axle housing replaced the 9-inch, and the 8.8-inch saved about 50 pounds and boasted an 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. 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 plugwelded 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 tubeto 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.
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. These housings are still highly sought after and can be pricey to acquire. Aftermarket units are available to help out with this shortage. There are even companies making 12-bolt housings to fit non-GM muscle cars.
While I have provided a fair amount of information to digest so you have a clear idea of the most axle options on the market, I have not provided an exhaustive account of all axles out there. Armed with this information, you should be able to spot the differences and tell one axle from another when you’re sifting through the scrap yard. And you should be able to make informed purchasing decisions so you select the right axle for your particular vehicle.
Written by Joe Palazzolo and Posted with Permission of CarTechBooks