Feeler gauges are precision tools for determining the width of a narrow space. Although the most familiar use of a feeler gauge is setting the spark plug gap for gasoline engines, all machines have moving parts that must fit together according to design tolerances.
The feeler gauge does not measure the gap; instead, it performs a "Go-NoGo" test. If the blade of a gauge fits into a gap, the gap is at least that wide. If it doesn't fit, the gap is narrower than the blade's thickness. Using the gauge properly generally requires multiple tests.
Determine the proper width of the gap to be measured from the specifications for the machine or device.
Select the blade of the specified thickness from among the blades on a feeler gauge. Each blade will be marked with its thickness in inches, millimetres or both. Inspect the blade; if it is damaged or corroded, discard the feeler gauge and get a new one.
Slide the blade of the gauge into the gap. If it will not slide in, the gap must be widened. If it slides in easily, attempt to insert the next thicker blade in the set. If that blade will slide in, the gap must be narrowed.
Adjust the width of the gap until a blade of specified thickness just slips into the gap and the next larger blade does not fit.
Feeler gauges should be kept lightly oiled to prevent rust from developing on the blades. When gapping a spark plug, use a gauge with round wire feelers instead of blades. The procedure is the same.
Never attempt to set a gap on a machine while it is running.