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Reading an outside micrometer to thousandths

Aviation Mechanic General (AMG) practice question 23 of 23 on Inspection Concepts and Techniques. ACS element AM.I.K.K1 · 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 measures a part with an outside micrometer. The last visible figure on the horizontal line of the barrel is 2, three additional 0.025 inch graduations are exposed past that figure, and the 12 graduation on the bevel edge of the thimble coincides with the horizontal line. What is the measurement?

  1. A0.212 inch
  2. B0.275 inch, with the thimble adding nothing
  3. C0.287 inch
Show the answer and explanation

Correct answer: C. 0.287 inch

  • A — incorrect This total leaves out the three exposed 0.025 inch barrel graduations, which are worth 0.075 inch and must be added to the reading.
  • B — incorrect The thimble graduation that coincides with the horizontal line is always part of the reading, and each thimble division is worth 0.001 inch.
  • C — correct The figure 2 on the barrel represents 0.200 inch, the three exposed graduations add 3 x 0.025 = 0.075 inch, and the thimble adds 12 x 0.001 = 0.012 inch, for a total of 0.287 inch.

BACKGROUND

An outside micrometer is read by adding three values taken from its scales. The numbered lines on the barrel mark tenths of an inch (0.100, 0.200, 0.300, and so on), and each space between those numbers is divided into four parts of 0.025 inch each, because one full turn of the thimble moves the spindle 0.025 inch. The bevel edge of the thimble carries 25 equal divisions, so each thimble division represents one one-thousandth (0.001) of an inch. To read the instrument, note the last visible tenths figure on the barrel, add 0.025 inch for each barrel graduation showing past it, then add the thimble division that lines up with the horizontal line on the barrel. Micrometers fitted with a vernier scale further divide 0.001 inch into ten parts of 0.0001 inch each.

STUDY TIP

Practice the three-step addition until it is automatic: barrel tenths, then 0.025 inch per exposed barrel graduation, then 0.001 inch per thimble division. Most wrong answers on this topic come from skipping the barrel graduations between the numbered tenths.

KEY POINTS

  • Numbered barrel lines are tenths of an inch; each smaller barrel graduation is 0.025 inch.
  • Each of the 25 thimble divisions equals 0.001 inch, and one full thimble revolution moves the spindle 0.025 inch.
  • A micrometer reading is the sum of the barrel tenths figure, the exposed 0.025 inch graduations, and the aligned thimble division.

SOURCES

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