FAA Aviation Mechanic Powerplant (AMP)Engine InspectionHard
A mechanic is performing internal timing on a magneto and rotates the drive shaft to the E-gap position before closing the breaker points. What does the E-gap position represent?
- AThe point where the magneto is producing zero output voltage
- BThe point where the distributor gear is fully meshed with the drive gear
- CThe point just past neutral where the rate of change of magnetic flux is maximum, producing the strongest spark
- DThe point where the rotating magnet is at its neutral (zero flux) position
Show answer & explanationAnswer & explanation
Correct answer: C. The point just past neutral where the rate of change of magnetic flux is maximum, producing the strongest spark
The E-gap (efficiency gap) is the angular position of the rotating magnet just beyond its neutral point where the rate of change of flux—and therefore the induced voltage in the primary coil—is at its maximum, ensuring the strongest possible spark when the points open.
Why the other options are wrong
- A. Zero output voltage does not correspond to the optimum spark-producing position.
- B. Gear meshing relates to distributor timing, not the E-gap concept.
- D. The neutral position is where flux is at maximum but the rate of change is zero, not ideal for point opening.
Magneto E-Gap Timing
The E-gap is the small angular displacement past the rotating magnet's neutral position at which breaker points should open, producing the maximum rate of flux change and strongest spark.
- E-gap stands for 'efficiency gap'
- Occurs a few degrees after the magnet's neutral position
- Breaker points must open exactly at E-gap for correct internal timing
- Improper E-gap timing reduces spark energy and can cause hard starting/misfires
Memory trick: 'Past Neutral, Peak Power' — E-gap is just after neutral where voltage peaks.