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.