FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsEasy
A technician is setting the electrode gap on a new spark plug for a reciprocating engine cylinder to a specification of 0.016 inch. Which tool provides the most accurate gap measurement for this round-electrode plug?
- AA vernier caliper
- BA round wire spark plug gap gauge
- CA dial indicator
- DA flat feeler gauge blade
Show answer & explanationAnswer & explanation
Correct answer: B. A round wire spark plug gap gauge
A round wire gauge matches the curvature of round spark plug electrodes and contacts them at the correct points, giving an accurate gap reading. A flat feeler blade only contacts the high points of curved electrodes and can give a false, tighter-than-actual reading.
Why the other options are wrong
- A. A caliper is not designed to measure the small precision spark gap.
- C. A dial indicator measures displacement, not electrode spacing.
- D. Flat blades contact curved electrodes only at high points, giving inaccurate readings.
Spark Plug Gap Measurement
The distance between center and ground electrodes, measured with a round wire gauge matched to the electrode shape for accuracy.
- Round wire gauges match curved electrodes
- Flat feeler blades give false readings on round electrodes
- Gap must meet manufacturer spec (commonly 0.012–0.019 in.)
- Improper gap affects spark energy and plug life
Memory trick: Round plug, round tool — match the shape to get the truth.