Which instrument should be used to measure finished diameters?

• A. Micrometers
• B. Sine bar
• C. Snap gauge
• D. Hooked scales

Answer: (A)

Measuring a finished outside diameter requires an instrument that can contact the outside of the part with high precision and give a clear, repeatable reading. A micrometer does this best because it is designed specifically for external dimensions, with a calibrated sleeve and a rotating thimble that yield the diameter to the smallest increment your tool allows. The ratchet or friction mechanism helps apply consistent pressure, so readings aren’t influenced by how hard you squeeze. Before using it, zero the tool on the same measuring range and ensure the part and tool are clean. Place the part between the anvil and spindle, bring into light contact, then read the measurement. A sine bar is used for angles, not diameters; a snap gauge can be quick but isn’t as precise or flexible for finished diameters; hooked scales are for rough or alternative measurements and don’t provide the same level of accuracy for external dimensions.