Feeler Gauges are used to precisely determine the size of small gaps between two objects that are in close proximity and usually consist of several finger-shaped strips of thin metal which vary in thickness (also known as leaves or blades), secured at one end by a pivot pin.
Metric and imperial versions are available, each individual finger is etched with the thickness of the finger. Multiple thicknesses can be achieved by layering several fingers together to reach the thickness you require. If the gap is larger than the blade, and there is little or no resistance, change to a thicker gauge or add additional blades until the resistance feel is correct.
Feeler gauges are traditionally used by mechanics for the fine setting or verification of clearances between two parts, such as spark plug gaps, ignition ‘points’ on a petrol engine or engine