What was found
ENGINE S/N L3049651E EXPERIENCED A STICKING EXHAUST VALVE AT 582.4 HOURS TOTAL TIME WHILE IN FLIGHT. THE PILOT MADE A SUCCESSFUL PARTIAL-POWER LANDING. CYLINDER NR 3 EXHAUST VALVE STUCK IN FLIGHT, THE VALVE HIT THE PISTON AND THE PUSH ROD WAS BENT. THAT CYLINDER WAS REPLACED WITH A NEW ONE. WHEN REAMING THE REMAINING EXHAUST VALVE GUIDES, ALSO FOUND CYLINDER NR 4 STICKING. THIS WAS REAMED OUT AS WERE THE OTHER 2 EXHAUST VALVE GUIDES. LYCOMING SB388C WAS PERFORMED ALL GUIDES WERE REAMED TO WITHIN BALANCE. THE ENGINE INCORPORATES NI-RESIST VALVE GUIDES AND THE TIME TO FAILURE IS CONSIDERED BY THIS AGENCY TO BE PREMATURE.