What was found
APPROXIMATELY 2 MILES FROM AN AIR FIELD, THE ENGINE BEGAN TO RUN ROUGH. LANDED WITHOUT INCIDENT. INSPECTION REVEALED THE NR 4 CYLINDER EXHAUST ROCKER ARM HAD CRACKED AND SEPARATED, ON THE BOTTOM INBOARD SIDE, IN THE BUSHING AREA. THE BUSHING WAS NOT DAMAGED. NO OTHER PART IN THE CYLINDER WAS DAMAGED. INSTALLED A RE-CERTIFIED ROCKER ARM. RUN-UP AND TEST FLIGHT CHECKED OK.