SHIM

A spacer is a part that sets and maintains a precise distance or gap between two components.

Fitted between parts it positions them correctly and takes up load, setting clearances such as bearing preload or alignment; thin spacers used to fine-tune a gap are called shims.

Spacers and shims are used in hubs, gearboxes, differentials, and many assemblies where clearance matters. The correct thickness is set to the specified clearance.