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Micrometer reading from barrel and thimble

Aviation Mechanic General (AMG) practice question 32 of 36 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

QUESTIONA mechanic reading an outside micrometer finds that the last visible figure on the barrel is 3, that three additional 0.025 inch graduations are exposed beyond that figure, and that the 8 graduation on the thimble coincides with the horizontal line on the barrel. What is the measurement?

  1. A0.383 inch
  2. B0.3758 inch
  3. C0.375 inch
Show the answer and explanation

Correct answer: A. 0.383 inch

  • A — correct The figure 3 on the barrel represents 0.300 inch, the three exposed graduations add 3 x 0.025 = 0.075 inch, and the thimble division adds 0.008 inch, for a total of 0.383 inch.
  • B — incorrect This treats each thimble division as one ten-thousandth of an inch, but each of the 25 divisions on the bevel edge of the thimble represents one one-thousandth (0.001) of an inch.
  • C — incorrect This is the barrel reading of 0.300 inch plus the three 0.025 inch graduations only; the thimble division that lines up with the barrel must also be added.

BACKGROUND

On an outside micrometer, the lines on the barrel marked 1, 2, 3, 4, and so forth indicate 0.100 inch, 0.200 inch, 0.300 inch, and 0.400 inch respectively. Each section between those tenths divisions is divided into four parts of 0.025 inch each, and one complete revolution of the thimble moves it one of these 0.025 inch divisions along the barrel. The bevel edge of the thimble is divided into 25 equal parts, so each thimble division represents one one-thousandth (0.001) of an inch. The micrometer is read by noting the last visible figure on the barrel, adding the number of exposed graduations multiplied by 0.025 inch, and then adding the thimble division that coincides with the line of graduation. The total of the three figures equals the measurement.

STUDY TIP

Practice the three-step addition until it is automatic: tenths from the numbered barrel line, then 0.025 inch for each exposed graduation, then thousandths from the thimble. All four types of micrometer (outside, inside, depth, and thread) are read the same way.

KEY POINTS

  • Numbered barrel lines are tenths of an inch; each smaller barrel graduation is 0.025 inch.
  • One full turn of the thimble advances 0.025 inch, and the thimble's 25 divisions are 0.001 inch each.
  • Add barrel tenths, exposed 0.025 inch graduations, and the coinciding thimble division to get the reading.

SOURCES

  • FAA-H-8083-30B ch11 "Reading a Micrometer"
  • FAA-H-8083-30B ch11 "Micrometer Parts"
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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.