Crate Axle Confessional -- Technical Article -- Chevy High Performance Magazine
Exploring the moser 12-Bolt
From the February, 2009 issue of Chevy High Performance
By Steve Magnante
Man, have we got it good these days. Never before in the history of Chevy hot- rodding--and restoration--have we had access to so many brand-new reproduction parts from the aftermarket. The days of scrounging and refurbishing hopelessly worn-out vintage junk are just about over, unless you're caught up in the date-code sadist/masochist cycle.
Today's racer/restorer can now order a complete, fully assembled 12-bolt "crate axle" from Moser Engineering that's all-new and superior in every way to OE GM axle assemblies. Moser sells 'em with "ears" for coil-spring cars or with pads for leaf-spring cars. Best of all, with prices starting at $1,768, it is often less expensive to pick up the phone than it is to tackle the task of rebuilding and upgrading a blown-up Chevy 12-bolt.
To demonstrate the point, we enlisted Mike Morgan, whose primered Quick and Dirty '69 Nova has been a regular "little guy" hero car in magazines for several years, with its impressive 9-second quarter-mile performance and Mike's ability to do it--reliably--on a shoestring budget. Go check beside the throne in your reading room; it's in our Feb. '02 issue.
But even Morgan, a self-proclaimed cheapskate, recognized the false economy associated with rebuilding his 12-bolt after the ring gear scattered some teeth and brought his fun to a halt. Even if he coughed the coins to install new gears, how long until the same problem happened again? Clearly, he'd reached the strength threshold and upgrades were needed. So he bit the bullet and made the call to Moser, who advertises a two-day turnaround on most orders. Sure enough, ours arrived about six days after we hung up the phone (four days for surface transportation).
Though Moser crate axles are shipped fully assembled and ready for vehicle installation, we wanted to show what you get for your money. To do this we asked Jim Cook of Performance Differential to disassemble our Moser crate axle for a look-see. Let's follow along as Jim explores the many ways Moser has refined and strengthened the classic Chevy 12-bolt rear axle.
When Mike's Nova started "making...
When Mike's Nova started "making crunchy sounds," he pulled the inspection cover on the stock 12-bolt and discovered some loose gear teeth.
Stock GM carrier-bearing caps...
Stock GM carrier-bearing caps are secured by 7/16-inch bolts and often have noticeable casting porosity that can lead to failure. Notice how the pen points out the pits. If the surface looks like this, what does the core metal look like?
By contrast, Moser caps are...
By contrast, Moser caps are improved in every way. They're made of high- nodular-content iron with a uniform thickness, are 0.950 inch wide (versus 0.750) for better carrier bearing support, use larger 1/2-inch Grade 8 bolts, and are 0.640 inch thick (versus 0.560) for increased resistance to deflection. The stock-diameter 3.062-inch carrier bearings are plenty strong and are retained.
The stock carrier-bearing...
The stock carrier-bearing journals are skimpy, web-like structures and are drilled through (note the end of the pen is visible). This limits the amount of thread engagement possible and reduces the cap's ability to resist uprooting forces when heavy power is transmitted from the pinion gear into the ring gear.
This is the same area on the...
This is the same area on the Moser case. Notice how the carrier-bearing journal is machined into a beefy shelf, which is wider than stock to better support the carrier bearings. The carrier-bearing bolt-holes are threaded deeply into the journals for maximum bolt-thread engagement and strength.
We made random audits to check...
We made random audits to check the thickness of the Moser case and found it to be 15-20-percent thicker than stock. The added thickness equates to extra rigidity and distortion resistance. An example of the added thickness is seen here at the inspection cover flange, where the micrometer reads 0.450 inch thick (versus 0.360).
Unlike stock axle tubes, which...
Unlike stock axle tubes, which are pressed into the case then secured by a few small plug welds, Moser tubes are fully welded into the case. Also, the case ends are 1/4 inch thicker than stock (3.875 versus 3.650) and deeper to permit greater tube engagement. This distributes torsional stress over a larger area of the tubes and eliminates the classic 12-bolt tube failure problem.
The stock axle breather vent...
The stock axle breather vent location is not ideal. Jim Cook says he sees similar GM 14-bolt truck axles with 4.10:1 gears generate enough internal splash effect during extended highway driving that they lose oil, overheat, and fail.
Though the Moser case is cast...
Though the Moser case is cast with an un-drilled vent boss provision, crate axles are shipped with a breather hole tapped into the axle tube. After the installation of a thread-in breather, the axle is safe from leak-inducing gear splash. The standard Moser 12-bolt has seamless low-carbon axle tubes with 0.188-inch wall thickness (versus 0.150).
The Moser crate axle comes...
The Moser crate axle comes standard with a bulletproof Spicer 1350-series pinion yoke and 3/8-inch U-bolts. This is the same size used on 1-ton trucks and will take just about any power level you can dish out. For those preferring not to rework their stock driveshaft, Moser offers a new 1310 yoke as a $35 credit.
With all the added strength...
With all the added strength benefits, you'd expect a significant weight penalty. Not so; the bare Moser unit weighs 83 pounds, a mere 7-pound increase, even with the optional extra-heavy 0.250-inch axle tubes. The only guys who sweat unsprung weight increases like this are hardcore road racers. Street and strip users will never know the difference.
Even at 30 splines, the 12-bolt's...
Even at 30 splines, the 12-bolt's greatest weakness is the lame direct-acting bearing, c-clip- axle arrangement. The only thing this stuff is good for is keeping production cost low when you're GM and you've got to build millions of axles for cheap. If you're a die-hard, Moser does offer high-grade 30- and 33-spline c-clip axle shafts that are NHRA-legal.
We specified Ford "big bearing"...
We specified Ford "big bearing" tube ends and sealed axle bearings. Available in a few flavors, our tube ends are designed to accept Chevy drum brake backing plates so Mike could reuse his stock 10-inch Nova drums. While we were at it, we also upgraded to 33-spline axle shafts, a no-cost upgrade if a spool is specified (limited-slip customers pay a small added charge for 33-spline compatibility). Be sure to tell Moser what brakes you plan on using so you get the correct axle flange offset.
The stock Chevy brake backing...
The stock Chevy brake backing plate has a 2.7-inch center hole. To accommodate the Ford axle bearings and retainer it must be enlarged to 3.150 inches with a die grinder. This hassle can be skipped by ordering Ford drum brakes (with the Chevy bolt pattern, of course), but Mike had just rebuilt his Nova's drums and couldn't bear to toss them.
Mike runs Centerline rims,...
Mike runs Centerline rims, so he specified the optional 5/8-inch-long wheel studs for $80. The stock studs are puny 7/16 twigs.
Mike selected a 33-spline...
Mike selected a 33-spline Moser spool (left) instead of a clutch-type Posi-Traction unit. The spool suits the hard-charging Nova's need for uniform power delivery to both slicks. Buyers have their choice of either type of differential for the same money, though 33-spline customers will pay a little more for a 33-spline limited-slip unit.
The optional cast-aluminum...
The optional cast-aluminum inspection cover ($135) features pre-load adjusters that bear gently against the carrier-bearing caps to prevent deflection and gear-mesh irregularity. The pre-load is set to 5 ft-lb. Dig the handy fill and drain plugs. Before ordering the special cover, make sure you have adequate gas tank clearance.
Our thanks to Jim Cook and...
Our thanks to Jim Cook and Performance Differential for providing disassembly and reassembly services. Jim's ready to help you with your rear axle parts and service needs.
OE Fortification: False Economy?
If you're maybe thinking you can get it done cheaper the old-fashioned way, here's a ballpark cost analysis of what it takes to refurbish and fortify a stock GM 12-bolt rear axle. The total amount is within striking distance of the Moser base price. As for the shipping charges, contact Moser for specific pricing, but rest assured you may spend more running all over town rounding up parts if you attempt this in do-it-yourself mode. Think it over, then make the call!
*Used 12-bolt case housing: $100 to $250 (when you can find one)
*Re-tube 12-bolt case to your width: $379
*Custom axle tube ends: $80
*Custom alloy axle shafts: $315
*Differential setup kit: $100
*1350-series pinion yoke: $130
*Upgraded wheel studs: $80
*Setup labor: $125
TOTAL = $1,645 to $1,795