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Traction Action

Building a Detroit Truetrac-Equipped 12-bolt Currie Axle
By Scott Crouse
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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.
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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.
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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.
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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.
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Our Currie technician fastened the ring gear to the Detroit Truetrac with 50 lb-ft of clamping force.
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With the pinion gear standing on the press, the pinion bearing was pressed into position.
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The differential with carrier bearing installed must be carefully set in place using special shims to locate the ring gear properly against the pinion.
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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.
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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.
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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.
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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.
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To keep the lube inside our 12-bolt, Currie installed a pinion seal by gently tapping it into place around the pinion.

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