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Opposed engine firing in pairs

Aviation Mechanic Powerplant (AMP) practice question 4 of 13 on Reciprocating Engines. ACS element AM.III.A.K4 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONWhy can the firing order of an opposed engine usually be listed in pairs of cylinders?

  1. AEach pair fires across the center main bearing
  2. BEach pair shares one common intake manifold pipe
  3. CEach pair is driven by a single camshaft lobe
Show the answer and explanation

Correct answer: A. Each pair fires across the center main bearing

  • A — correct The handbook states that opposed engine firing order can usually be listed in pairs because each pair fires across the center main bearing.
  • B — incorrect Sharing an induction pipe is not the reason given for the paired listing; the pairing comes from firing across the center main bearing.
  • C — incorrect Cam lobe sharing relates to valve operation, not to why the firing order is expressed in pairs.

BACKGROUND

The opposed-type engine has two banks of cylinders directly opposite one another with a crankshaft in the center, and the pistons of both banks connect to that single crankshaft. Firing order is the sequence in which the power event occurs in the different cylinders, chosen to provide balance and reduce vibration. In opposed engines the order can usually be listed in pairs of cylinders because each pair fires across the center main bearing. The firing order of six-cylinder opposed engines is 1-4-5-2-3-6, while one model of four-cylinder opposed engine uses 1-4-2-3 and another uses 1-3-2-4.

STUDY TIP

Memorize 1-4-5-2-3-6 for the six-cylinder opposed engine, and remember that cylinder numbering is not standardized, so the engine manual decides which cylinder is No. 1.

KEY POINTS

  • Opposed firing orders are listed in pairs because each pair fires across the center main bearing.
  • The six-cylinder opposed firing order is 1-4-5-2-3-6.
  • Opposed cylinder numbering varies by manufacturer, so consult the engine manual.

SOURCES

  • FAA-H-8083-32B ch01 "Firing Order"
  • FAA-H-8083-32B ch01 "Opposed or O-Type Engines"
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MORE ON RECIPROCATING ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.