If a mechanic needs to confirm the inside diameter of a small hole, which pair of tools is appropriate?

• A. A small hole gauge and a micrometer caliper.
• B. Dial indicator and depth gauge.
• C. Vernier caliper and outside micrometer.
• D. Radius gauge and feeler gauge.

Answer: (A)

To confirm the inside diameter of a small hole, you need a tool that goes inside the bore and a precise way to read that internal size. A small hole gauge is designed to be inserted into the hole and expanded until it just fits, establishing the bore’s size. To read that internal size accurately, you then use a micrometer caliper, which provides high-precision measurements when reading the gauge. This combination gives a reliable internal measurement.

The other tool pairs don’t fit this need. Dial indicators and depth gauges measure displacement and depth, not the actual internal diameter. A vernier caliper paired with an outside micrometer is geared toward external dimensions, and while a vernier caliper can measure inside with its small jaws, it isn’t as precise for small bores as using a hole gauge with a micrometer. Radius gauges and feeler gauges measure radii or gaps, not a direct, precise bore diameter.