It's built strong, lasts long, and hooks the tires so hard it's just plain wrong. This is a fair description of Detroit Locker's new Detroit Truetrac locking differential. Whether it's an 8.5-inch ring gear 10-bolt rearend, or a heavy-duty 8.875-inch 12-bolt bad boy, the Truetrac differential is capable of lowering acceleration times without excessive wear to internal components or the rear tires. We wanted to see what makes the Truetrac design tick, and the guys at Currie Enterprises offered to take us through a 12-bolt buildup and the benefits of a Detroit Truetrac locking differential.
Today's world of aftermarket bliss offers so many types of differentials it's difficult to determine which is best for your needs. Straight-line acceleration lends itself to a spool type for absolute positive axle power, while high-performance street/strip lockers fall within an overwhelming sea of choices. The key to a street-style locking differential is how to compensate for varied wheel speed around corners, while also being able to deliver constant and equal drive force to each drive axle when straight-line acceleration is needed. Addressing these issues are some of the most popular locking-differential designs featuring spring tension-induced cones, friction plates, air pressure, and electrically operated setups. Each of these works well for a specific application, but also requires some sort of maintenance or additional support system. With the use of a Truetrac differential design, Detroit Locker has been able to provide a wear-free, gear-driven, limited-slip unit.
The Detroit Truetrac limited-slip differential features a patented design of parallel-axis planetary-helical gears that split torque with such ease there is little driveline disturbance from open-differential action to engaged-axle drive. The helical gears require no air tanks to mount, wires to cross, or friction components to replace. It provides a wear-free environment that is capable of handling strong doses of torque from heavy-hitter street/strip engines. By eliminating conventional clamping-friction components inside the differential, the helical gear design is capable of handling much more torque capacity. When combining the affordability of a Truetrac differential design with characteristics of wear-free maintenance and loads of additional torque-carrying capacity, it's easy to see why a helical gear-driven posi is so popular.
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 Our starting point was the...  Our starting point was the famous 12-bolt rearend. General Motors produced Chevrolet 12-bolt passenger car rearends for many '65-72 models, depending on application. |
 Currie cut off the housing...  Currie cut off the housing ends of our '69 Chevelle axle and replaced them with 9-inch style ends. This, along with relocating the shock brackets, allows the interchange of more than 30 different OE and aftermarket brake applications. |
 Truetrac differentials use...  Truetrac differentials use parallel-axis planetary-helical gears to split torque, and can be set-up with or without the C-Clips; Currie installed ours without. The 9-inch ends hold the axles in place, while a steel shim internally keeps the axles spaced evenly apart. |
 Our converted 9-inch housing...  Our converted 9-inch housing ends require the axles to be equipped with a lock collar, bearing, seal, and hangar plate to support the axle inside the housing. The seals feature pre-load tabs that compensate for variations in the housing ends and brake-backing plates. |
 Our Currie technician fastened...  Our Currie technician fastened the ring gear to the Detroit Truetrac with 50 lb-ft of clamping force. |
 With the pinion gear standing...  With the pinion gear standing on the press, the pinion bearing was pressed into position. |
 The differential with carrier...  The differential with carrier bearing installed must be carefully set in place using special shims to locate the ring gear properly against the pinion. |
 Once the pinion shim was installed...  Once the pinion shim was installed and the bearing was pressed over the shaft, the entire assembly could be placed inside the housing. This was where things got tricky--the pinion shim thickness required a guess as to how far in or out the pinion-gear teeth would ride against the ring gear. If the guess was wrong, everything would have to be done again. |
 From the pinion head back,...  From the pinion head back, a depth shim and bearing fit over the 30-spline shaft and ride against a bearing race inside the housing. From there, a pre-load spacer was placed over the pinion shaft, where it was either crushed or shimmed in place as the outer bearing and yoke were tightened against it. |
 The standard car yoke size...  The standard car yoke size is a 1310-series that measures 3 1/2 by 1 1/16 inches. The next-step-up 1330-series yoke measures 3 5/8 by 1 1/16 inches, and the strongest series, the 1350 (image), measures 3 5/8 by 1 3/16 inches. |
 With all shims in place and...  With all shims in place and our differential fastened securely, we checked the backlash of the ring gear in relation to the pinion gear. An acceptable amount of lash is 0.008-0.012 inch. |
 To keep the lube inside our...  To keep the lube inside our 12-bolt, Currie installed a pinion seal by gently tapping it into place around the pinion. |