Internal Timing Without Special Tools

Achieving precise internal timing is typically done using a specialized plate and pointer tool. In the absence of this equipment, improvisation is necessary. Using a drafting protractor, I marked 10, 15, and 25-degree increments directly onto the casing, while a simple straight-edge served as a makeshift pointer. With the timing gear’s red mark visible, the armature is rotated backward until the magnets reach a “neutral position.” Although this tactile feedback is subtler when parts are lubricated, the red tooth remains a reliable visual reference point.

For the timing process, the magneto is rotated in its normal direction until the 10-degree mark aligns with the pointer. Since professional timing lights were unavailable, I employed a classic technician’s hack: adjusting the points until they just barely release a thin piece of cellophane. Once set, the magneto is rotated further to verify the maximum gap, ideally measuring 0.018 ± 0.006 inches.


The Role of the Impulse Coupling

The impulse coupling is critical for engine starting, as it provides a robust spark and retards timing at low cranking speeds to prevent dangerous engine kickback. The mechanism works as follows:

  • At low RPMs, cams engage studs to briefly hold the rotating magnet.
  • As the drive gear turns, internal ramps retract the cams.
  • The sudden release of the spring snaps the magnet forward, generating a high-energy spark.

Once the engine achieves sufficient speed, centrifugal force keeps the cam lobes disengaged from the studs, allowing for standard ignition timing—typically 25 degrees before Top Dead Center (TDC).


Troubleshooting and Final Assembly

After reassembly, testing is essential. I successfully verified spark generation by driving the armature with a cordless drill. A critical discovery during this phase involved the carbon brush in the rotor:

«It was compressing down and getting stuck in the retracted position instead of extending up and contacting the coil.»
This seemingly minor issue prevented the magneto from firing, serving as a reminder that even new components must be verified.


Key Takeaways for Builders

Reflecting on the rebuild process, several best practices emerged for those attempting their own maintenance:

  • Documentation: Take extensive photos during disassembly to ensure correct reassembly.
  • Preservation: Never discard old parts immediately; they serve as vital references for ordering replacements.
  • Patience: Use proper tools like two-jaw pullers for flyweights and avoid over-tightening gear nuts, which can prevent the impulse coupling from functioning.
  • Cleanliness: Proper cleaning is a significant portion of the work, given the buildup of aged grease and debris.

Ultimately, rebuilding these units provides a cost-effective alternative to buying new, and with careful attention to detail, these vintage magnetos remain a reliable power source for experimental aircraft.