As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE UPON RETURN TO BASE ENG DROPPED ABOUT 300 RPM AND BEGAN TO RUN ROUGH, LANDING WITHOUT INCIDENT. UPON INVESTIGATION EXHAUST VALVE FACE HAD BROKEN INTO 2 PIECES AND VALVE STEM WAS BENT AND HAD BROKEN JUST BELOW THE TIP WITH ALL 3 PIECES BEING FOUND IN EXHAUST. REMAINING TIP OF VALVE STEM ALONG WITH KEEPERS WERE STILL INSTALLED IN VALVE SPRING SEAT. PISTON CROWN AND CYL HEAD SHOWED EVIDENCE OF DAMAGE DUE TO CONTACT WITH THE PIECES OF VALVE.