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Reading an outside micrometer

Aviation Mechanic General (AMG) practice question 24 of 33 on Aircraft Materials, Hardware, and Processes. ACS element AM.I.E.K6 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONOn an outside micrometer, the last visible figure on the barrel is 2, three more barrel graduations are visible beyond it, and the 12th graduation on the thimble is aligned with the barrel line. What is the measurement?

  1. A0.275 inch
  2. B0.212 inch, ignoring the small barrel graduations
  3. C0.287 inch
Show the answer and explanation

Correct answer: C. 0.287 inch

  • A — incorrect This is the barrel reading alone (0.200 + 0.075). The 0.012 inch shown on the thimble must be added.
  • B — incorrect The three 0.025-inch barrel graduations beyond the figure 2 cannot be skipped; they add 0.075 inch to the reading.
  • C — correct The figure 2 gives 0.200 inch, three graduations of 0.025 inch add 0.075 inch, and the 12th thimble graduation adds 0.012 inch, for a total of 0.287 inch.

BACKGROUND

The lines on the micrometer barrel numbered 1, 2, 3, and so on indicate tenths of an inch (0.100, 0.200, 0.300 inch). Each space between these numbers is divided into four parts of 0.025 inch, and one full turn of the thimble moves it one such division. The bevel edge of the thimble is divided into 25 parts, each representing 0.001 inch, and these divisions are numbered at every five spaces (0, 5, 10, 15, 20). To read the micrometer, note the last visible tenth figure on the barrel, add the number of visible 0.025-inch graduations beyond it multiplied by 0.025 inch, and then add the thimble division that coincides with the barrel line. The total of the three figures is the measurement.

STUDY TIP

Always add three numbers: the tenth figure, the extra 0.025-inch graduations, and the thimble reading.

KEY POINTS

  • Barrel numbers are tenths of an inch; the small graduations are 0.025 inch each.
  • Each thimble graduation is 0.001 inch; 25 of them equal one barrel graduation.
  • The reading is the sum of barrel tenths, barrel graduations, and thimble divisions.

SOURCES

  • FAA-H-8083-30B ch11 "Reading a Micrometer"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.