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Inside diameter calculation from tube number and wall thickness

Aviation Mechanic General (AMG) practice question 9 of 17 on Fluid Lines and Fittings. ACS element AM.I.D.K2 · 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 piece of number 6 rigid tubing has a wall thickness of 0.035 inch printed on it. What is the inside diameter of the tube?

  1. A0.340 inch
  2. B0.3125 inch (5/16 inch)
  3. C0.305 inch
Show the answer and explanation

Correct answer: C. 0.305 inch

  • A — incorrect This subtracts the wall thickness only once; the wall is on both sides of the bore, so twice the wall thickness must be subtracted from the outside diameter.
  • B — incorrect This is simply the next smaller tube size (number 5); it ignores the wall thickness entirely.
  • C — correct Number 6 tubing has an outside diameter of 6/16 inch (0.375 inch). Subtracting twice the wall thickness, 0.375 - 2(0.035) = 0.305 inch.

BACKGROUND

Metal tubing is sized by outside diameter (OD), measured fractionally in sixteenths of an inch, so number 6 tubing is 6/16 inch (3/8 inch) and number 8 tubing is 8/16 inch (1/2 inch). The tube diameter is printed on all rigid tubing, and the wall thickness is printed in thousandths of an inch. Because tubing is made in various wall thicknesses, a mechanic must know the material, the OD and the wall thickness when installing tubing. To find the inside diameter (ID), subtract twice the wall thickness from the outside diameter. The handbook's own example is a number 10 tube (0.625 inch) with a 0.063 inch wall, giving an ID of 0.499 inch.

STUDY TIP

Convert the dash or tube number to sixteenths first, then subtract the wall thickness twice; forgetting the second wall is the classic error.

KEY POINTS

  • Tube size numbers are the outside diameter in sixteenths of an inch.
  • Wall thickness is printed on the tubing in thousandths of an inch.
  • ID = OD - 2 x wall thickness.

SOURCES

  • FAA-H-8083-30B ch09 "Sizes"
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MORE ON FLUID LINES AND FITTINGS

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.