As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MAST BEARING INNER RACE HAS BECOME DEBONDED FROM THE MAIN ROTOR SHAFT. MAIN ROTOR SHAFT WAS REMOVED AND SENT FOR REPAIR. INSTALLED SERVICEABLE SHAFT TO RETURN ACFT TO SERVICE. DO NOT KNOW WHAT HAS CAUSED THE INNER RACE OF THE MAST BEARING TO DE-BOND AND SPIN ON THE MAIN ROTOR SHAFT. THIS IS THE THIRD ACFT WE HAVE INSPECTED WITH 600 TO 800 HOURS TIME IN SERVICE AND FOUND THE BEARING INNER RACE DE-BONDED.