A feeler-gauge kit consists of blades of varying thicknesses used mostly for measuring the gap between contact-breaker points and sparkplug electrodes, and for checking valve clearances. Gauges measure in either imperial or metric sizes; most car service manuals give an alternative for either system. The range of measurements covered is usually between 0.001 and 0.025 in. or 0.05-0.80mm. Make sure you buy a gauge suitable for the clearances on your car.

A sparkplug gapping tool has stiffer, thicker blades (or wires) than a feeler gauge, and is more likely to give an accurate measurement for a sparkplug gap. It also has hooks for levering the electrode. On some cars, the sparkplug gap may be larger than the range of sizes on a feeler gauge - 0.04in. (1mm) for example. Although you can use two or three feeler blades together, a gapping tool is preferable.

A feeler gauge is a tool used to measure gap widths. Feeler gauges are mostly used in engineering to measure the clearance between two parts.

They consist of a number of small lengths of steel of different thicknesses with measurements marked on each piece. They are flexible enough that, even if they are all on the same hinge, several can be stacked together to gauge intermediate values. It is common to have two sets for imperial units (typically measured in thousandths of an inch) and metric (typically measured in hundredths of a millimetre) measurements.

A similar device with wires of specific diameter instead of flat blades is used to set the gap in spark plugs 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.

The lengths of steel are sometimes called leaves or blades, although they have no sharp edge.

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