Definition
In a magneto, the number of degrees the rotating magnet has turned past its neutral position at the moment the breaker points open to fire the spark plug. At this angle, the magnetic flux in the magneto's iron core is changing at its maximum rate, producing the strongest possible voltage in the secondary coil for ignition.
Plain English
It is the small angle past the magnet's neutral spot where the magneto is set to fire the spark. Firing exactly at this angle gives the hottest, strongest spark.
Context Anchor
Seen when checking or adjusting the internal timing of an aircraft magneto, usually during maintenance rather than normal cockpit use.
Derivation
The 'E' stands for 'efficiency.' It marks the angle at which the magneto produces its most efficient (strongest) spark. Calling it the 'E-gap' angle is shorthand for 'the efficiency gap angle' between the magnet's neutral position and the point where the points open.
Why Pilots Care
Correct E-gap angle produces reliable, high-energy sparks; an incorrect setting causes weak ignition, hard starting, and possible engine roughness.
Grounding Statement
Inside the magneto, the spark is strongest only during a small part of the magnet rotor's travel.
Intuition Check
Do not think of E-gap as a physical air gap to be measured with a feeler gauge. Here, gap means an angle of rotation inside the magneto.
Example Sentence 1
When timing the magneto to the engine, the technician first sets the magnet to the E-gap angle before adjusting the breaker points.
Example Sentence 2
An E-gap angle set too early or late reduces spark intensity and can cause starting problems on a cold engine.