When installing the pinion, there are two ways to set the bearing preload. One is to use a crush sleeve (left). With this method a non-reusable crush tube is collapsed between the bearings when the assembly is tightened. The second method uses a solid cast-iron spacer (right). Obviously, the solid spacer does not collapse, so shims are used with the solid spacer to accurately set the bearing preload. Shims are added and subtracted to get the proper preload. There are advantages and disadvantages to both methods. The solid spacer takes more effort to install, but is considered more stout. Should the pinion flange take a very hard hit on a rock or other immovable object, it is possible for a crush sleeve to be impacted and crushed slightly more. When this happens, it leaves some freeplay in the bearings, and the assembly loosens slightly.