Which tool is commonly used to measure the diameter of a crankshaft journal?

• A. Torque wrench
• B. Micrometer
• C. Dial gauge
• D. Vernier caliper

Answer: (B)

You measure a crankshaft journal with a tool built for precision external dimensions. A micrometer provides the fine, repeatable accuracy needed to read small diameters like a journal. When you place the journal between the micrometer’s spindle and anvil and gently tighten, the spindle moves in tiny, calibrated steps, letting you read the diameter to the thousandths of an inch (or hundredths of a millimeter). For crank journals, you’ll often take several measurements around the circumference to confirm the surface is true and within tolerance, catching any out-of-round wear.

The other tools aren’t as well suited for this specific task. A torque wrench is for applying a specific torque, not measuring size. A dial gauge measures displacement or runout, which helps assess wobble but doesn’t give a direct diameter reading. A Vernier caliper can measure external dimensions, but its precision is typically lower than a micrometer for small, tight tolerances like a journal, and it’s easier to introduce measurement error on curved, precise surfaces. So, the micrometer is the best choice for accurately measuring a crank journal’s diameter.