What tool should a mechanic use to accurately measure a hole approximately 1/4 inch in diameter?

Using a small-hole gauge combined with a micrometer is the most appropriate choice for accurately measuring a hole approximately 1/4 inch in diameter. The small-hole gauge is specifically designed to fit into small openings and can help determine the diameter of the hole. Once the gauge is inserted, it can be adjusted to fit snugly against the walls of the hole. After fitting the small-hole gauge, the measurement can be taken with a micrometer, which provides a precise reading due to its ability to measure in fine increments. This combination ensures both accuracy and reliability when measuring small diameters.

Other tools mentioned may not provide the same level of precision. A caliper is beneficial for depth measurement but may not be as effective for measuring small diameters directly as a small-hole gauge. A standard ruler, while useful for rough estimates, lacks the precision needed for close measurements of such small sizes. A digital gauge, although instant and convenient, may not be suitable for very small holes compared to a dedicated small-hole gauge and micrometer. Overall, the combination of a small-hole gauge and micrometer is designed specifically for accurate measurements in these situations.