feeler gauge is a tool used to check and measure narrow gaps and
clearances between two parts. A feeler gauge consists of a number
of small lengths of steel (sometimes called blades) of different
thicknesses with measurements marked on each piece. The blades are
hinged at one end and fold into a steel case. It's common to have
two sets of feeler gauges, one for metric units and the other for
imperial measurements, although sets are available that have both
metric and imperial size blades and some have both measurements
marked on each blade. Gradations are typically from fractions of
a millimeter up to a few millimeters or fractions of an inch. They're
used by finding one that fits smoothly in the gap being measured.
Sometimes the best fit is found by using a combination of blades.
The blades are flexible enough that several can be stacked together
to gauge intermediate values. The measurement is the total thickness
of all the gauges that fit snugly into the gap.
A similar device with wires of specific diameter instead of flat
blades is used to set the gap in spark plus to the correct size.
This is done by increasing or decreasing the gap until the gauge
of the correct size just fits inside the gap.