How to read a metric feeler gauge

Written by brad chacos
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How to read a metric feeler gauge
Mechanics use feeler gauges to check spark plugs. (spark image by Zbigniew Nowak from

Machinists, mechanics and home-remodelling enthusiasts use feeler gauges to measure tight-fitting gaps between two surfaces. Although a standard caliper is used for most measurements in many settings, occasionally a gap is too small or difficult to access with a caliper. Feeler gauges typically come in sets of a certain range of sizes. The gauges themselves are flat pieces of finely calibrated metal blades. Each blade's thickness measurement is stamped directly on the blade itself. Feeler gauges come in both metric and English versions. You'll need to calculate the conversion if using a metric feeler gauge for English measurements.

Skill level:

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Things you need

  • Pencil and paper or calculator

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  1. 1

    Remove a blade from the feeler gauge.

  2. 2

    Look for a measurement value stamped on the blade. The measurement on metric feeler gauges is typically to hundredths or thousandths of a millimetre and signified by an "mm" stamp next to the measurement.

  3. 3

    Convert the number to its English counterpart if necessary. Multiply the millimetre value by 25.4 to determine the size of the blade in inches.

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