As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING A 100HR INSP, A CRACK WAS DISCOVERED COMING FROM THE LOWER SPARK PLUG HOLE PROCEDING TOWARDS THE EXHAUST VALVE. IT IS COMMON PRACTICE IN THE FACILITY TO BORESCOPE THE CYLINDERS AT EACH 100 HR INSP TO LOOK FOR SUCH CRACKS AS PREVENTITIVE MX TO REDUCE THE NR OF IN FLIGHT FAILURES.