If you need to check a valve stem for stretch, which instrument is appropriate?

• A. Micrometer
• B. Caliper
• C. Dial indicator
• D. Depth gauge

Answer: (A)

Precise measurement of small external dimensions is being tested. A valve stem is a small cylindrical part, so you need an instrument that can read tiny changes in its outside size with high accuracy. A micrometer fits this need because it uses a calibrated screw and a spindle that gently contacts the stem to give a direct measurement in thousandths of an inch (or hundredths of a millimeter). That fine resolution lets you detect even small stretch or wear.

A caliper can measure the stem but isn’t as precise, especially for tiny features. A dial indicator measures displacement or runout, not the actual size of the stem. A depth gauge measures depth, not the stem’s diameter. So for confirming stretch on a valve stem, the micrometer is the best tool.