What was found
DURING DESCENT FROM ALTITUDE AFTER A 2 HOUR CROSS COUNTRY FLIGHT AND 10 MILES FROM HOME BASE, THE ENGINE BEGAN TO RUN ROUGH. DESCENT WAS STOPPED AT 2.500 FEET TILL A SAFE LANDING WAS ASSURED. THE ENGINE, ALTHOUGH RUNNING ROUGH, RAN WITH GOOD OIL PRESSURE AND NORMAL OIL TEMPERATURE. EMERGENCY CHECKS DISCOUNTED IGNITION AND FUEL PROBLEMS. SUSPECTED A STUCK OR BROKEN VALVE. A SAFE LANDING WAS MADE WITHOUT INCIDENT. UPON INSPECTION, THE NR 2 CYLINDER SPARK PLUG SHOWED SIGNS OF MECHANICAL DAMAGE. THE CYLINDER WAS REMOVED AND CONFIRMED THAT APPROXIMATELY ONE-THIRD OF THE EXHAUST VALVE HEAD HAD BROKEN OFF. THE BROKEN LOOSE PART DID CONSIDERABLE DAMAGE TO THE TOP OF THE CYLINDER HEAD, PISTON, AND SPARK PLUGS.