Bulletproofing a Chevelle 12-bolt with Currie Enterprises


GM’s 8.875-inch, 12-bolt positraction rearends came in most 1964 to '72 GM performance cars. The pinion shaft on passenger car versions is 1.625 inches in diameter, and the carrier cover is oval, measuring 10.9375 by 10.625 inches. The differential carrier is attached to the differential assembly using two main caps. Light truck applications used a different version of the 12-bolt and the two are not interchangeable. The truck diff has a smaller, 1.438-inch diameter pinion shaft and the rear cover measures 10.875 inches by 10.875 inches. They came in both open and positraction configurations, and are now difficult to find and highly prized by muscle car and restoration enthusiasts. Performance versions of the following GM models all came with 12-bolt rearends that were generally adequate to service the torque output of period muscle car engines.
1965-'70 Impala with 4-link suspension
1965-'72 Chevelle, Malibu, El Camino and GMC Sprint
1970-'71 Monte Carlo
1970 Buick Special and Pontiac Firebird
1968 and 1969 Olds F85 and Cutlass (some) with "C"-type axles.
1971 Nova
1970-'72 Tempest with type "C"-type axles
1970 and 1972 Grand Prix
Full-size passenger cars and A-body and G-body mid-size cars, used rear coil springs. Early Camaro and Nova rear axles used leaf springs. They only interchange within their respective body lines due to the different suspension mounts. The project car for this build is a 1970Chevelle originally equipped with a 12-bolt that had seen better days.We chose to take advantage of Currie Enterprises superior upgraded 12-bolt assembly to gain the advantage of OEM style disc brakes and top quality internals. The following photo sequence details the step-by-step build of Currie specific 12-bolt for a coil spring application. These assemblies are hand-built by experienced technicians who do it every day so you can be sure its done right.
Leaving nothing to chance, Currie casts its own nodular iron housing for 12-bolt center sections. The casting is robust with additional reinforcement to ensure optimum strength and trouble-free operation in performance applications. It features a world class finish and is made to exact specifications to accept all standard Series 3 and Series 4 12-bolt GM and aftermarket internals. The build technician begins by measuring the inside diameter of the axle housing openings. Then left and right side axle tubes are cut the the desired length. The axle tubes are  3-inch by .188 wall DOM steel (.250-inch optional). The small hole on top of the housing is the initial weld point for the axle tube.


Precision housing measurements are every bit as important to your rearend assembly as blueprinting is to your performance engine. Currie blueprints rearends for the same reason. Perfect alignment of the axle splines inside the gear carrier ensures that all the splines carry an equal load without binding and applying undue stress to the axle and carrier bearings and the gearset.
The inside diameter of the axle tube is machined to accept a standard dimension Ford housing end and the outside diameter is machined to seat fully in the center housing with a .005-inch interference fit.
The center housing is heated in an oven to 450 degrees to expand the axle tube openings. A technician then applies a special lube to the axle tubes and they drop into place in the expanded housing.
A rigid bar is inserted through the entire assembly to maintain straightness while welding the axle tubes and housing ends.
Precision machining and interference fit ensure correct alignment in the center housing. Currie's builders are top tier welders and assemblers. Have a look at those welds and imagine that on a custom Currie rearend in your hot rod.
Another jig bolted to the center housing positions and aligns the trailing arm brackets and spring perches for welding. Different jigs are used depending on whether the rearend is for a leaf spring car or a coil spring application.  Custom positioning can also be accommodated providing that accurate measurements are supplied.
The spring perch is leveled and tacked in position prior to final welding. The jig positions the trailing arm mounts at the proper angle for the car. When welding is completed the entire assembly is given a hot bath to clean it prior to send it to the assembly station.
When you opt for a Currie 12-bolt, you're getting much more than a brand new housing with top quality parts. The C-clip axle locks are eliminated in all Currie 12-bolts and the axle tubes are outfitted with new Ford style housing ends that have larger axle bearings that permit the use of factory stock Ford Explorer rear disc brakes. This feature provides disc brake performance with easily serviceable factory parts when maintenance is required. For pure restoration applications, the axle tubes can be equipped with stock style housing ends that accept drum brakes. The same quality and  build content goes into every unit whether it's for a leaf spring application or for coil springs.
Twelve -bolt housings for coil spring cars are fitted with a Currie exclusive Johnny Joint® housing kit. These joints provide smooth and predictable suspension movement without the bind associated with stiff urethane bushings.
Ford style axle tube housing ends make axle installation, retention and maintenance  much easier and they provide the correct mounting points to accept disc brakes from a Ford Explorer.


A new differential carrier housing is used in every build. Currie builds with either Eaton or, as shown here, Yukon differential kits. As the customer, we suppied a Yukon Dura Grip kit, PN YDGGM12P-3-30-1 and Currie technicians set it up. The kit comes with a fully assembled positraction unit of your choice (3 or 4 series), new carrier and pinion bearings, a shim kit for accurate setup, a pinion seal, carrier bolts rear cover gasket and attending hardware. The 3.73 gearset we used is PN YG GM12P-373. The technician always lays the parts out on the bench to verify all the parts are there before he starts the build.
Red thread sealer is used to anchor the fasteners on the ring gear. The gear is pulled evenly onto the carrier and the bolts are torqued t0 65 lb-ft using an alternating sequence.
The carrier bearings are driven on with a hammer and alignment tool. Then the pinion shaft is polished to accept the pinion bearing that is installed with the hydraulic press.


The bearing race for the front pinion is installed and the carrier bearing shim pack is installed on each side.The shim pack adjusts the carrier bearing preload. Using an inch-pound torque wrench on the pinion nut, the carrier should require 7-10 in-lb. more to rotate than it does with just the pinion gear by itself.
The pinion gear, crush sleeve and pinion yoke are installed with a preliminary shim pack which amounts to a well-educated guess. The pinion nut is tightened to compress the crush sleeve the desired amount.
The bearing races are then slipped over the carrier bearings and the differential assembly is lowered into the case merging the ring gear with the pinion gear for the initial clearance and pattern check.
Checking the backlash is an important step to ensure that a proper amount pf play exists between the pinion gear and the ring gear.
The gear teeth are painted with a lead-based gear checking compound that will reveal the gear engagement pattern when the assembly is rotated through its travel.
The tech makes appropriate adjustments to the shim packs to position the largest portion of the contact patch just ahead of the center of the gear teeth on the drive side. The pinion shims control the depth of the pinion engagement with the ring gear. That's how the pattern is centered on the gear teeth so it is not too close to the top or bottom. The carrier shims control the depth of gear engagement so the pattern is fully engaged, but no so much that its binds up. This photo shows near perfect engagement right out of the box because Currie techs know from lengthy experience what is required.

During the setup process, the tecnician is striving for a balanced pattern which provides not just optimum engagement strength, but also offers longer gear life and reduced noise. The correct pattern is like an elongated oval that slightly favors the toe side of the ring gear on the drive face. Under power, the pattern "stretches" as load is applied equally across the gear teeth. Favoring the toe side helps ensure that the pattern doesn't run off the heel end under load, flirting with potential gear breakage.

On aftermarket gear sets, the pinion gears are etched with a gear set number. The recommended pinion depth is etched next to the set number. Pinion depth is the distance from the rear of the pinion to the centerline of the carrier. You will also find a set number on the ring gear along with a backlash spec. Currie techs work with these specifications and adjust accordingly if necessary.

Once the correct pattern and backlash are established, red thread locker is applied to the carrier cap bolts and they are torqued to  75 lb-ft. Currie makes its own high strength caps as part of their in-house center section.
The cross shaft anchors the differential gears inside the positraction unit. Thread locker is applied to the cross shaft lock screw to keep it secure. The bolt is torqued to 25 lb-ft.


Axles are splined to allow for varying lengths as required. Once a requirement has been determined they are cut to length.
L to R. The shiny ring on this axle indicates the depth of the heat treat. Currie adapted an old press to provide a quick means of pressing in the axle studs. A standard press is used to install the axle bearing. Between the two springs, you can just make out the end of a tape measure hooked over the back of the side gear for axle length measurement.
The axle length to housing flange distance is recorded and the correct length axles are selected from pre-fabricated axles.


All mounting points for suspension components and brake hardware are attached to the axle housing with high-quality welds. This makes the assembly a true plug-and-play unit.
Ford Explorer rear disc brake set with integral drum parking brake are a bolt-on with the attending Ford housing ends.
All new hardware in the Ford Explorer brake kit assures top quality and performance when adding disc brakes to your Currie/GM 12-bolt.
Explorer disc brakes make maintenance and parts replacement as close as any parts store or dealership.
An important component of Currie rear axle assemblies is attention to detail. Every rearend is equipped with all the requisite mounting brackets and hardware to fashion a complete working assembly. Rearends are provided with the proper lubricant and positraction additive. No other parts necessary.
Currie Enterprises
382 N Smith Ave.
Corona, CA 92880
(714) 528-6957