As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED A STRONG VIBRATION IN FLIGHT. MAINTENANCE INSPECTED AND FOUND THE LEFT ENGINE #1 CYLINDER TO HAVE LOW COMPRESSION AND THE #3 AND #4 CYLINDERS TO HAVE DAMAGE AT THE ROTOCOIL. ALL THREE CYLINDERS WERE REPLACED WITH NO FURTHER DEFECTS NOTED.