Let's face it, performance cars are all about discovering the weak link in the chain. As soon as you start making serious horsepower, what happens? You break a transmission or a rearend because it's too weak. Darwin taught us that only the strong survive. This means that you can forget the spindly 7.5- and 8.2-inch 10-bolts. They're cannon fodder. For this story, we're going to compare the 12-bolt to the much-vaunted Ford 9-inch to evaluate the advantages and disadvantages of both rearends. To help us with our quest, we contacted Randy's Ring and Pinion to track all the variables.
The 12-bolt and 9-inch rears are actually more similar than they are different. The major difference between the two is the design of the housings. The 12-bolt integral design features a carrier case and pinion support that are part of the rearend housing, while the 9-inch employs a drop-out third member that takes advantage of an independent carrier case and pinion support that bolts into the rearend housing.
12-Bolts to Glory The GM 12-bolt debuted in 1965 and was mass-produced until 1972. It features an 8.875-inch ring-gear carrier held in place with two internal main caps. The pinion gearshaft measures 1.625 inches in diameter and is shimmed to achieve the proper pinion depth. The pinion gear splines to a yoke that mates with the driveshaft's rear universal joint. The three most common types of yokes are the 1310-, 1330-, and 1350-series units, but the 12-bolt generally uses the smallest, 1310-style U-joint.
Factory axles come in a 30-spline configuration and slide into the side gears. Instead of bolting the axles in place, stock GM >> 10- and 12-bolts use small C-clips over the ends of the axles to hold them in place. This requires removing the rear cover, draining the oil, and extracting the center pin in order to pull the C-clips to remove the axles (whew!). Axle bearings are pressed into the tube with the outside diameter of the axleshaft acting as the inner bearing brace. The downside to this idea is that if the bearing fails, it takes out the axle too.
The 12-bolt is more complicated to assemble because it uses a shim between the pinion gear and the inboard pinion bearing. Since this bearing is pressed on, it must be removed with a press to change the pinion shim to adjust the pinion depth. Also, the ring-gear backlash is adjusted with shims as well. This requires the proper thickness of shims on both sides to ensure the carrier bearings are preloaded and that the backlash is correct. All of this is not difficult but does require specialty tools to accomplish. While original 12-bolt housings are becoming rare, Moser has stepped into the void with a brand-new housing configured for all the popular Chevy body styles.
When the 12-bolt rearend is properly assembled, it's the strongest passenger-vehicle rearend Chevy ever produced. GM engineers designed the 12-bolt to be a durable piece that could handle the torque output of its big displacement engines.
The Big 9-Inch
The 9-inch Ford rearend has established itself as the high-water mark for rear axle strength and durability. The 9-inch has become so pervasive in the performance market that you can purchase bolt-in 9-inch housings from companies like Currie Enterprises and Randy's Ring and Pinion for any Chevrolet body style you can conjure up.
The 9-inch rearend was first mass-produced by Ford Motor Company in 1957 and continued for 30 years. While the 9-inch uses many similar pieces to the 12-bolt, it configures them in a somewhat more convenient and durable package. A 9-inch rearend features a stamped housing with nothing more than axletubes welded on each end. This type of rearend is easier to service because of its removable centersection that houses the gearset. This removable gear case houses a differential unit that the ring gear bolts onto. The 9-inch uses a bolt-on external pinion support that locates the large pinion bearing. But an added plus for the 9-inch is an internal rear-pinion support that also supports the gear end of the pinion to limit gear deflection under high torque loads.
Like the 12-bolt, the 9-inch pinion gear is shimmed to achieve the proper pinion depth with a small pinion shaft diameter of 1.313 inches. Another difference between the two rearends is that the 9-inch locates its pinion gear lower on the ring gear to improve tooth contact. This strengthens the rearend assembly but at the cost of approximately 2 to 3 percent loss of power to drive the gears compared to a 12-bolt. Of course, the sizes of the gears, cases, and bearings are relevant to strength, so you have to give the strength comparison to the 9-inch with its 0.125-inch larger ring-gear diameter and internal pinion support.
Another advantage to the 9-inch is that the axles are easier to install and remove. Ford designed the factory 28- and 31-spline axles to be removable by yanking four bolts per axle flange, which allows the axle to slide out. The axles have pressed-on bearings and retainer plates that bolt to the housing flanges. There are no C-clips inside the rearend holding them in place like on a 12-bolt. The plus here for the 9-inch is that if you break an axle inside the housing, the axle retainer will keep the axle in place. On a 12-bolt, breaking an axle means it slides right out of the housing, causing massive sheetmetal damage along the way. You can convert a 12-bolt to the 9-inch style of axle retention, but it requires a C-clip eliminator kit and modifications to the housing.
Ultimately, the selection of 9-inch vs. 12-bolt comes down to personal preference. If you are building a cruiser and want to keep everything GM, the 12-bolt is more than capable of taking the abuse. On the other hand, if you plan to flog your Bow Tie machine with slicks and lots of torque, it's hard to beat the 9-inch's strength and ease of service. Both are great pieces, so we'll let you decide.
 A complete 12-bolt rearend...  A complete 12-bolt rearend with drum brakes weighs around 185 pounds. It uses 2 to 3 percent less power than a 9-inch and even factory pieces are capable of withstanding 500 hp using 30-spline axles. |
 The typical 9-inch rearend...  The typical 9-inch rearend weighs in around 177 pounds (without brakes). A benefit of this design is that its third member can be easily accessed, and the factory components are capable of withstanding even more torque with 31-spline axles. |
 The three most common types...  The three most common types of yokes are the 1310-, 1330-, and 1350-series units. While factory 12-bolt rearends usually come with a 1310-style yoke, it's always a good idea to step up the size. The Randy's Ring and Pinion third member on the right is equipped with a larger 1330-series yoke. |
 One of the major differences...  One of the major differences between a 12-bolt and a 9-inch is that that the 9-inch offsets the pinion to the passenger side, creating unequal-length axles. These are a pair of 31-spline, 9-inch axles. |
 From left to right you can...  From left to right you can see how the carrier unit and ring gear bolt to the case mains. Completing the third member is a pinion gear that's shimmed for proper gear-tooth contact and held firmly by a pinion support and driveshaft yoke. |
 The weak link on a factory...  The weak link on a factory 9-inch gear case is the inner pinion support. |
 Notice the additional material...  Notice the additional material around the pinion support on the 9+ race case. This factory case broke due to multiple high-torque load applications--clutch dumps. |
 If you are planning on racing...  If you are planning on racing with a 9-inch rearend, the 9+ case is the way to go. Randy's can build a complete centersection to your individual needs. This unit features a 9+ race case, an anodized-aluminum pinion support, a forged yoke, a Detroit locker carrier unit, and a set of 4.11 gears. |
 Companies like Randy's, Moser,...  Companies like Randy's, Moser, and Currie also offer complete 12-bolts ready to race. These rearends are virtually indestructible and employ thick-wall tubing, reinforced main caps, top-quality carrier units, and almost any rearend ratio you can imagine. The 12-bolt housings can also be ordered with 9-inch axle ends to eliminate the weak C-clip design. |
 Not all drop-out Ford rearends...  Not all drop-out Ford rearends are 9-inch units. The easiest way to identify them is by examining the two lowest gear-case bolts. If the bolts are covered so that only a wrench will fit over them, it's a 9-inch. If you can get a socket over these bolts, it's not. |
 One disadvantage to the 9-inch...  One disadvantage to the 9-inch is its weight. One way to lighten the assembly is with an aluminum centersection like this one from Strange. Strange also offers a slick aluminum centersection designed to use 12-bolt gears and internal parts. |