Flex in a housing assembly can be one of the most detrimental forces on a rearend. According to Moser, a common problem with stock 10- and 12-bolt rearends is gear deflection. In simple terms, this is when force delivered by the pinion gear pushes against the ring gear and tries to push it out the back of the housing. The purpose of the main caps is to transfer the climbing force exerted on the ring-and-pinion into a horizontal force to the side gears, which in turn rotates the axles. When grip and torque exceed the capacity of the main caps, they can break or stretch, resulting in ring and pinion failure. "Rear covers add a kind of doorstop to the caps by giving them extra leverage against these forces and allow the rearend to handle a greater torque load," says Moser.
Another common problem is the flexing of the axle tubes. "Wider rearends impart more leverage and stress on the housing and centersection. Increasing horsepower and vehicle weight also increases the likelihood of rearend flex, misalignment, and subsequent failure of internal parts. Securing the tubes to the housing with a back brace dramatically increases the strength of the housing and will afford greater consistency when running at the track."
One of the benefits of running a 9-inch rearend is the myriad configurations in which they can be arranged. When it comes time to select a case, the options are either a standard cast, nodular iron, or aluminum third member. Moser says that a standard cast iron case is good for applications of 400 hp or less. While you may be able to get away with more power, the pinion support is prone to failure with high-rpm launches and sticky tires. Nodular and aluminum cases are a big step up in strength, but choosing between the two depends on vehicle weight. "If a car is a daily driver with a full interior, then you might as well save your money and just go with a nodular case. If it is a stripped-down car and you're trying to save every ounce of weight, then an aluminum case might be a good choice," he says. "The one thing to look out for with an aluminum cases is that you really should have a through-bolt design on the main caps. Without a through-bolt design, the case can flex and cause ring gear deflection. We only offer our aluminum cases with through-bolts, and they're good for some very serious power."
Stock vs. Aftermarket Housings
Although they may be hard to spot with an untrained eye, aftermarket rearend housings incorporate several improvements over their stock counterparts. All of Moser's housings are manufactured from all-new housing centers and feature new brackets, housing ends, and larger axle tubes that are always welded to the center. "On our new 12-bolt rears we have increased the main cap size and use larger Allen-head bolts to secure the caps," Moser says. "The casting itself also has more ribbing to reduce deflection. Likewise, our standard 9-inch centers are new stampings that are much stronger than a stock piece."
C-Clip Eliminators and Housing End
If the thought of your wheels, tires, and axles ejecting themselves from the rearend housing is a bit unsettling, C-clip eliminators or weld-on housing ends are a cheap insurance policy. Moser says that C-clip eliminators are generally for cars that are being slowly converted to a full drag race setup. They work by acting as a safety hub with sealed, press-on bearings. In the event that an axle breaks at the spline, C-clip eliminators will keep them safely in the housing. However, they do have some drawbacks. "They weren't really designed with street driving in mind and can sometimes allow gear lube to leak through them," explains Moser. "We have made a number of design changes to our C-clip eliminators through the years to help eliminate this issue. The next step up from C-clip eliminators is weld-on housing ends and the conversion to press-on axle bearings. The weld-on housing ends obviously provide the safety needed for racing and can also handle the side loads of a street vehicle."