What instrument is best for measuring the inside width of a keyseat or the depth of a keyseat?

• A. Dial caliper
• B. Micrometer
• C. Vernier caliper
• D. Ruler

Answer: (A)

Measuring inside width and depth requires an instrument that can reach into the groove and give a clear, precise reading. A dial caliper is ideal here because it has internal jaws that fit into the inside of the keyseat to measure its width, and a depth measuring rod that extends from the end to reach the bottom of the seat. The dial readout provides a direct, easy-to-interpret measurement, which helps ensure the keyseat fits the key to spec. Ruler is too crude for the small dimensions involved; a micrometer mainly measures external dimensions (and has limited utility for inside measurements unless you use special accessories); a vernier caliper can do internal measurements but requires careful interpretation of the vernier scale, making the dial caliper the quicker, more reliable choice in many shop settings.