Feeler gauges are used to measure the distance between the two surfaces or objects and are most often seen in the automotive industry. They are typically made from uniform strips of metal of a precise thickness. A feeler gauge can come in either a single strip, or a set of strips that are hinged on the same screw or pivot pin, allowing you to manually adjust the thickness to a desired measurement. The name comes from the way they are used. A feeler gauge is inserted between the gap and moved around until resistance is encountered without forcing the surfaces apart, or binding with either of the surfaces that the gauge can no longer be moved about. There are several kinds of feeler gauges; however the two most common variants are the wire feeler and the flat feeler. Each is designed to address the kind of gaps it is meant to measure.

A feeler gauge is a staple in an auto mechanic’s tool bag and is used to measure gaps between contact points in many of the older engines or between spark plugs. These kinds of gauges are used because of the precise and repeatable measurements they provide. Repeatability of these measurements is important, especially in the automotive industry and automotive repair. For example, an automotive mechanic can use a feeler gauge to fine tune the gap between the contact points of an older engine’s ignition system. This kind of measurement ensures the continued operability of the vehicle. They can also be used in the same way to measure the gap between spark plugs, although a flat feeler gauge is typically used for engines, while wire feelers are used for measuring the distance between spark plugs and NPT pipe thread. Another way of using a feeler is as a temporary shim. Machines rely on shims to maintain correct alignment and not just distance. A feeler can be used as a temporary shim in this case to determine the correct alignment so the machine is assembled properly.

Aside from the common flat feelers and wire feelers, feeler gauges can come in an altogether different arrangement or form. One can find a graduated feeler gauge where measurements are indicated along the surface or along the circumference if it is a circular feeler. These can be gradually inserted into the gap until resistance is felt and the measurement can be taken. Some professions even make use of spur of the moment feeler gauges all the time. Carpenters are known to do this when building outside decks, using nails of a specific thickness to space the planks before nailing or screwing them as part of the assembly. Doctors also use dental floss much in the same way as feeler gauges, using the floss to determine the ideal spacing between teeth or crowns.