Building a performance machine is all about making more power and then uncovering the weak areas that inevitably show up as a result of the upgrades-and usually at the worst possible moment. We've been beating on our mule '65 Chevelle for more years than we care to count, so it wasn't a big surprise when an axle broke right at the flange. We've subjected our Chevelle to much more than just high torque and sticky-tire drag launches; we've also abused the stock 12-bolt axle with years of 1g lateral laps on the road course. So the snapped axle made it clear it was time for improvements.
Previous experience had taught us that while a C-clip eliminator kit is a popular axle upgrade choice, we discovered that our Baer rear-disc-brake kit would not work with the pillow block that mounts the bearing. So we went looking for a better solution. Moser has been building brand-new 12-bolt housings for Chevelles, Camaros, Novas, and other Chevys for several years now. The company is also well known for its bolt-in 9-inch Ford housing conversion. We wanted to stick with our 12-bolt for several reasons, including its reduced weight, better gear arrangement, and also because we have a whole shelf of 12-bolt gears and parts to choose from.
 This is what happens when...  This is what happens when you try to road race on stock, 40-year-old factory axles. The axle broke just outboard of the axlehousing. Luckily our Baer rear-disc-brake caliper mount kept the wheel from tearing up our rear wheelwell. |  Here is a comparison of the...  Here is a comparison of the old 12-bolt axle bearing (in the housing) next to the Moser aftermarket 9-inch Ford bearing we will use. The larger bearing is capable of handling much more abuse without tearing up axles. | |
We noticed that Moser equips virtually all of its new 12-bolt housings with forged-steel, CNC-machined housing ends that retain the stock 10- and 12-bolt backing-plate bolt pattern but allow the use of a much larger axle bearing. When we compared the size of this new bearing to the stock 12-bolt bearing, we knew this was the way to go for our 12-bolt. This larger axle bearing combined with a much stronger aftermarket axle will add the strength and durability we need to stop worrying about breaking an axle when diving into a corner at full song. While accommodating the larger bearing, this new housing end also allows us to retain either the stock rear drum brakes or use a Baer rear-disc-brake caliper bracket.
The premise is pretty simple-cut off the stock housing end, weld on this new Moser end, and line everything up so there is no bind on the axles once everything bolts back together. While the theory may be simple, its execution requires serious housing modifications that are beyond our backyard bolt-on abilities.
Since we already had a 12-bolt that had been modified for a C-clip eliminator kit, which removes the stock 12-bolt axle bearing pressed into the housing, it was a simple decision to go ahead and modify this existing housing. Of course, if you don't have a 12-bolt, Moser offers these housings in various levels of assembly depending upon your needs.
 These are the Moser 30-spline...  These are the Moser 30-spline axles that complete the conversion. We measured the larger Moser axles at the radius to the axle flange and found them to be 25 percent larger than the stock axles, which is a 0.470-inch increase in diameter at this critical area. According to Moser, each 30-spline axle is capable of withstanding 6,200 lb-ft of torque per axle. |  | |
Once we had the housing ends and new axles from Moser, we took our pieces to Ed Sutton at Sutton Engineering in Industry, California. Sutton carefully cut the housing ends and welded on new ends using a fixture to ensure the housing was straight after all the welding was completed. This was also a great time to have the tubes fully welded into the centersection to build a little more strength into the housing.
Since this move required a complete disassembly of the rear gears, we also decided to upgrade our abused Eaton clutch-type Posi with a new Tractech Detroit Truetrac gear-style limited slip. The gear-style limited slip is designed to apply maximum torque to the wheel that can handle the additional power without spinning. This will allow us to apply much more power coming off the corner on an autocross or road course without spinning the unloaded tire.
We also decided to upgrade our 12-bolt with a couple of other extras, including an installation kit from Ratech that includes new bearings, seals, and shims to set up both the pinion depth and the backlash. We also use a Ratech solid pinion spacer instead of a crush sleeve to establish a more consistent pinion position relative to the ring gear. This requires a little more setup time, but the increased durability is well worth the additional effort.
 We took two of our 12-bolt...  We took two of our 12-bolt housings to Ed Sutton at Sutton Engineering in Industry, California, where he adroitly machined both housings and welded the ends in place using a mandrel to maintain perfect alignment. |  We also decided to improve...  We also decided to improve our 12-bolt's durability with a Ratech solid pinion spacer that replaces the crush sleeve. This solid spacer maintains a more accurate preload on the two opposing pinion bearings, which makes life easier on the pinion gear. We also used a complete Ratech ring-and-pinion installation kit. |  We actually converted two...  We actually converted two different 12-bolt housings to the Moser design and used Ratech rebuild kits for both that included pinion and carrier bearings plus other necessary components to complete the reassembly. We decided to retain the original clutch-style Posi in the second rearend, so we also upgraded the internals with Ratech pieces. We've discovered that the cross pin will wear against the side gears, causing clearance that reduces the preload on the clutches. Replacing the spider gears and center pin is a big step toward optimizing an Eaton Posi. |
 To complete our 12-bolt conversion,...  To complete our 12-bolt conversion, we also added a Detroit Truetrac gear-style limited-slip unit from Tractech. The gear-style limited slip offers several advantages over clutch-type units, including more consistent performance over a much longer period of use and abuse. | | |