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Mopar 8 3/4-Inch Rear Axle Assembly -

Swapping Gear Ratios Is One Of The Quickest Ways To A Big Change On Any Street Car, So Let's Show . . .

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Shannon Hudson's performance world is evolving. For years, he drove a lime-green '69 Valiant with the total drag racing package, right down to the Mickey Thompson ET Drag Radials and 4.11 gears. Outside the dragstrip, the car wasn't as much fun, especially on Los Angeles freeways where he'd have to spin his small-block to 3,600 rpm just to keep from getting run over. Because Hudson, the owner of Redline Gauge Works, is a car crafter whose taste had changed, the time had come to do something different. He decided to ditch the drag race package, put fat tires on all four corners, improve the A-Body's handling, and swap out those 4.11s for something more streetable. The first step was to perform the gear change, so he pulled his Valiant up on the rack at his buddy Dave Reeves' rearend shop, and we stopped by to document the conversion to a set of 3.55:1s. Here's how it all went down.

Rear Axle Assembly - The Buildup
The Mopar 8 3/4-inch rear axle assembly was Chrysler's midsize gear carrier system-smaller than the massive Dana 60 used in the Hemicars yet beefier than the pedestrian 8 1/4-inch. But not all 8 3/4-inch rears are the same, as there are three different casting numbers (found on the side of the case) based on pinion bearing size. This is important because parts, torque values, and even the pinion preload specs vary based on these case dimensions. Hudson's 489 centersection originated in a '70 Challenger using the largest bearings of the three casting numbers of 41, 42, and 89. Hudson had already contacted Randy's Ring & Pinion and ordered a set of 3.55:1 gears, a master rebuild kit, and stronger axles with a larger 5x4 1/2-inch wheel mounting bolt pattern for a set of Rocket wheels.

By the time we got to Reeves' shop, technician Dale Lee had already popped the axles and the 8 3/4-inch rear axle assembly centersection out of the housing, stripped the centersection, and had it sitting on the bench ready for assembly. One thing Lee did before we got there was mark both main caps with a center punch because they do not interchange. Lee also removed the bearing pressed on the pinion shaft to measure the shim thickness. The 4.11 gear had a 0.012-inch shim under the pinion along with 0.036 inch of shims underneath the pinion bearing race closest to the ring gear in the case. Reeves noted that it is common to find shims in both places.

Rear Axle Assembly - Pinion Gear Size Differences Reeves decided to use a 0.015-inch shim under the pinion to compensate for the change in size of the pinion gear. Deep gears like 4.11s employ a small pinion gear, while the 3.55's pinion is quite a bit larger. This changes the relationship of the pinion to the ring gear, necessitating a thicker pinion shim to move the pinion gear deeper-or closer to the ring gear centerline. Reeves chose to do it this way based on his experience, and we decided to pass that knowledge along with a chart of pinion shim thickness recommendations that should position the pinion close enough that you shouldn't have to measure pinion depth. Because of this, we did not run through the pinion depth procedure here, but you can find it at CarCraft.com.

PINION SHIM VS. RATIO CHART
GEAR RATIO SHIM THICKNESS
3.23 0.015
3.55 0.015
3.73 0.014
3.90 0.013
4.11 0.012
4.30 0.011
4.56 0.010

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