the.com/feeler gauge

means A tool made of thin metal blades of precise, graduated thicknesses, used to measure or check the width of narrow gaps, such as spark plug electrodes or valve clearances.

from The name is plainly descriptive: each thin blade is a 'feeler,' slipped into a gap to 'feel' how wide it is by trial — if it slides through with a slight drag, that's your measurement. 'Gauge' comes via French 'gauge/jauge,' a measuring standard, ultimately from a root tied to testing or verifying — the same lineage behind 'gauging' a barrel's capacity. The compound word itself is a workaday 19th/20th-century mechanic's coinage, born alongside the rise of precision engines that needed tolerances checked by touch rather than sight.