As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) PILOTS EXPERIENCED A ROUGH RUNNING MAGNETO DURING A GROUND RUN UP WHICH CHANGED TO A DEAD MAG. UPON RETURNING TO BASE MAINTENANCE REMOVED THE MAGNETO AND FOUND THAT THE MAIN ROTOR SHAFT WHERE THE ROTOR GEAR M3827 MOUNTS ONTO THE SHAFT WAS SHEARED OFF.