What kind of gauge should be used to measure the electrode gap in aircraft spark plugs?

• A. A flat-blade feeler gauge.
• B. A round wire gauge.
• C. A digital caliper.
• D. A micrometer.

Answer: (B)

The electrode gap is a small, circular opening between the center and ground electrodes, so the round wire gauge is the best choice because it measures that gap directly by using wires of precise small diameters. You test by trying wires of known diameters until one fits the gap with a light drag; that diameter matches the gap size within tolerance. This method is quick, repeatable, and specifically suited to the curved, narrow gap in spark plugs.

A flat-blade feeler gauge isn’t suitable because the gap isn’t a flat, slit-like space. A digital caliper or a micrometer isn’t ideal for this tiny, curved opening either and doesn’t provide the straightforward go/no-go check that a round wire gauge offers.