feeler gauge

British  

noun

1. a thin metal strip of known thickness used to measure a narrow gap or to set a gap between two parts

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Example Sentences

This used to be a more painstaking, time-consuming process, where Zabel inserted a “feeler gauge” and by trial and error determined how many thin layers of metal it took to plug each gap, then wrote down in a ledger the gap measurement at each bolt.

From Seattle Times