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Pinion bearing preload is a critical step in the differential overhaul process because if you get too heavy handed you have to backtrack and start over. Further, getting this procedure right sets the stage for each following step of reassembly. It should be noted from the get-go that there is a difference between the minimum torque value for the pinion nut and the pinion bearing preload. The process starts by securing the nut to the pre-described minimum torque, then measuring preload and tightening in small increments as required until the target preload is attained.
Pinion Installation Basics
Starting with a bare pumpkin, oil the front and rear pinion bearing surfaces in the diff housing then drive the pinion races into the housing using a pinion bearing race driver until they are fully seated. Clean and apply high quality silicone to the races. TIP Treat pinion bearings and bearing races as a mated pair? If you have to change out a pinion bearing remember to install new races as well. Install pre-oiled front pinion bearing and pinion seal. Follow that up with installing the pinion gear and crush sleeve. Next, install pinion the yolk onto pinion splines making sure not to damage the pinion seal when tapping the yoke into place. Finally, apply oil to the pinion nut washer or thrust surface of the pinion itself, put some thread locking compound on pinion threads or threads of the pinion nut, and hand start the nut.
Torque Value Versus Preload
Tighten the nut until you can feel resistance when turning the yoke back and forth, check torque load. Typical starting points are 55 lb-ft for 3/8-inch bolts/nuts, 75 lb-in for 7/16-inch bolt/nuts, and 125 lb-in for 1/2-inch nuts/bolts. TIP Tap the end of the pinion gear with a punch to ensure the races are fully seated, shake it to evaluate for excessive play. Once you’ve confirmed the nut is properly torqued check for preload. Using a dial-style inch-pound torque wrench spin the nut/pinion to get a running measurement of how much torque it takes to spin the pinion. We’re talking about the running friction or rotating friction of the assembly, not necessarily brake torque.
Check, tighten, and check the preload as you go and continue to tighten until you reach the desired/required pinion bearing preload for new bearings as indicated on the accompanying chart. Sneak up on the proper measurement, if you over-torque the pinion nut and get too much preload you will need to replace the crush sleeve and start again.
Preload Importance
As mentioned earlier, this is the first step of the journey, getting off on the right foot is all the more important. Stability is the end goal of proper preload. Getting this step right prevents the ring and pinion gear from moving in a vertical, horizontal, or diagonal manner. Going too far out of spec will entice gear movement under load when accelerating or in release of load under deceleration, resulting in the reduction of backlash, which causes undue wear, binding, obtrusive noise, and/or driveline vibrations.
