Pinion Bearing Preload
Pinion bearing preload is the tension placed on the pinion gears bearings. It is similar to the preload adjustment used on tapered wheel bearings. A collapsible spacer is used to obtain the desired amount of pressure or preload. This spacer is placed between the bearings and as the pinion nut is tightened the spacer is crushed to obtain the specified preload.
This preload prevents the sideways thrust that moves the pinion gear to the outside edge of the ring gear. The collapsible spacer acts as a spring to counter this motion. Some of the heavy duty and more demanding differentials have a solid spacer instead. This type of spacer must be the appropriate size to obtain proper preload.
A collapsible spacer cannot be over tightened and then backed off. This would leave the pinion bearings with no preload at all. It takes hundreds of pounds of torque to turn the pinion nut and can be tricky towards the end. Once the companion flange feels snug turn the pinion nut in 1/8 inch increments and then with each turn check the turning tension with an inch pound torque wrench.
These specifications are in inch pounds and vary with minimums like 15 inch pounds to maximums like 29 or 30 inch pounds. Once minimum torque is reached its not much more turning effort before the maximum allowed is reached. This reading is known as the turning torque.
