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Aircraft systems and instrumentsQuestion 579 of 934

Why do horizontally-opposed engines always have an even number of cylinders?

  1. Odd numbers of cylinders cannot be air cooled
  2. An even number is needed to keep the oil pressure
  3. Each magneto must fire two cylinders at once
  4. Each cylinder opposes one on the other side
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Correct answer: Each cylinder opposes one on the other side

A cylinder on one side of the crankcase "opposes" a cylinder on the other side.

Source: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 7: Aircraft Systems — Reciprocating Engines, page 162

Practice aircraft systems and instruments

Questions last reviewed: 18 October 2026