ring gauge, is a cylindrical ring of a thermally stable material, often steel, whose inside diameter is finished to gauge tolerance and is used for checking the external diameter of a cylindrical object.

Feeler gauges are used:
– To check the gap between the mating parts
– To check and set the spark plug gaps
– To set the clearance between the fixture (setting block) and the cutter/tool for machining the jobs
– To check and measure the bearing clearance, and for many other purposes where a specified clearance must be maintained.

Tolerance is defined as the difference between the maximum and the minimum sizes allowed in manufactured components.

Algebraic difference between a size and a corresponding basic size is called deviation. Limit deviations are upper and lower deviations. Algebraic difference of actual size with basic size is called actual deviation.
Upper Deviation: It is the algebraic difference between the maximum size and the basic size.
The upper deviation of a hole is represented by a symbol ES (Ecart Superior) and of a shaft, it is represented by es.
Lower Deviation: It is the algebraic difference between the minimum size and the basic size. The lower deviation of a hole is represented by a symbol EI (Ecart Inferior) and of a shaft, it is represented by ei.