Which instrument is used to measure the outside diameter of a rocker arm shaft?

• A. Micrometer caliper
• B. Dial indicator
• C. Vernier caliper
• D. Depth gauge

Answer: (A)

Measuring a small external dimension with high precision requires an instrument that directly contacts the outside surface and provides a fine, repeatable reading. An outside micrometer (micrometer caliper) is designed for that purpose: its screw-driven spindle and fixed anvil give very precise measurements of the shaft’s outer diameter, often to thousandths of an inch or hundredths of a millimeter. This level of accuracy is what you want for a rocker arm shaft, where tight tolerances matter.

A dial indicator is used to detect variations and runout, not to measure the diameter itself. A vernier caliper can measure outside dimensions, but its precision is typically lower than a micrometer, which is why it’s not the best choice for tight tolerances on a small shaft. A depth gauge measures depths, not diameters.

So, for accurately determining the outside diameter of a rocker arm shaft, the micrometer caliper is the best tool.