As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CRIMP NUT FOUND IN OIL COOLER WHEN FLUSHING DURING ENGINE CHANGE. REMOVED THERMOSTATIC BYPASS VALVE ON AFFECTED ENGINE AND FOUND NUT MISSING FROM VALVE. NO OIL TEMPERATURE PROBLEMS REPORTED BEFORE THIS. VALVE S/N IS 54849. LYC SB 518C ADDRESSES THIS PROBLEM ON THESE VALVES WITH SERIAL NUMBERS LOWER THAN 53788.