Feeler gauges are used to precisely determine the size of small gaps between two objects that are in close proximity.
Feeler gauges are typically used by mechanical engineers who require an accurate description of the clearance width between two parts. This is to ensure that mechanical parts are in safe working order.
All clearances vary depending on the parts in question and their application. You should set part clearances according to a vehicle manual or part specification to prevent damage occurring during operation.
A feeler gauge can be used to gauge a multitude of narrow slots. Multiple tests are usually required to confirm the exact size of a gap.