A feeler gauge is a measuring tool used to determine the clearance between two surfaces, such as a spark plug gap or a valve lash. It consists of a set of thin, flat blades of varying thicknesses, each with a marked size. To use a feeler gauge, insert the appropriate blade between the two surfaces and feel for resistance. If the blade fits snugly, the clearance is equal to the marked size of the blade.
Feeler gauges are an essential tool for any mechanic or engineer, and they are used in a wide variety of applications. They are relatively inexpensive and easy to use, making them a valuable addition to any toolbox.