As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE FLIGHT, ENGINE STARTED TO LOSE POWER AND BACKFIRE. PILOT MADE AN EMERGENCY LANDING IN FIELD WITH NO DAMAGE TO ACFT. VISUAL INSP OF THE ENGINE WAS ACCOMPLISHED. THE NR 6 CYLINDER WAS FOUND TO HAVE FAILURE OF EXHAUST ROCKER ARM BOSS RESULTING IN THE ROCKER ARM WRIST PIN TO BECOME DETACHED WITH PART OF THE ROCKER ARM BOSS ADHERED TO THE PIN AND UNABLE TO FUNCTION. THIS RESULTED IN THE EXHAUST PUSH ROD TUBE TO BECOME INEFFECTIVE CAUSING THE EXHAUST VALVE TO REMAIN CLOSED ALL OF THE TIME.