Which tool is best for checking the inside diameter of a hole for roundness?

• A. Bore gauge
• B. Outside micrometer
• C. Calipers
• D. Inside micrometer

Answer: (D)

Measuring inside dimensions with high precision is being tested. An inside micrometer is built for exactly that—checking the inside diameter of a hole with tight tolerance. It slides into the bore and the spindle and anvils give a precise reading on the micrometer scale. To assess roundness, you can take measurements at several orientations around the circumference; if the hole is perfectly round, these diameters will remain the same within tolerance. This tool provides the direct, high-precision inside measurements needed for evaluating circularity, which is harder to achieve with tools that aren’t designed for internal work.

A bore gauge can measure inside diameter, but it often relies on a stand or interpolation and is typically used for quick diameter checks rather than precise, multi-angle roundness assessment. An outside micrometer cannot reach inside a hole, and calipers, while usable for rough internal measurements, don’t offer the same precision or convenient multiple-angle readings needed to judge roundness accurately.