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Magneto TroubleShoot
Aircraft Magneto E Gap
The rotor magnets induce energy waves into the primary circuit only during certain intervals of their rotational cycle. The timing must be such that the energy wave induced in the coil's secondary windings is at its peak at the correct engine timing.
This is the point at which the magnetic circuit is saturated and capable of
producing the highest energy spark. Internal magneto timing is set by adjusting the breaker points so that they open at the correct rotor angular position.
This is called "E" gap.
E gap isn't a physical gap but an electrical gap. For instance, a 10 degree E gap means that the points break the electrical primary circuit at 10 degrees past the magnet's neutral position. Each time the
magnet is in the neutral position, the magnetic flux through the coil core is zero. When turning the magneto requires a timing light and usually a timing index and pointer to measure the number of degrees past neutral.  Slick
Aircraft Magneto showing timing pin
Setting point gap with safety wire gives incorrect E gap when the points or cam follower are worn. For example, cam follower wear causes E gap to increase from 8 to 10 degrees. A point gap of .018 inch now occurs further up the cam lobe now that the points have worn. E gap is reduced. Since
the aircraft magneto E gap has not been measured but approximated by setting point clearance, the first signs of trouble occur when the engine runs rough because of spark plug misfire.
If you encounter an enemy aircraft, you will
probably be a little busy. Patrol Sense US NAVY 1944
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