What was found
CREW REPORTED ENGINE ROUGHNESS IN FLIGHT AT POWER REDUCTION FOR CRUISE AND MIXTURE FULL RICH. THE ACFT LANDED SAFELY AT ITS INTENDED DESTINATION WITH NO ISSUE. INSPECTION DISCOVERED LOW DIFFERENTIAL COMPRESSION CHECK RESULT AT CYLINDER NR 4 OF THE LT ENGINE. A BORESCOPE INSPECTION WAS PERFORMED. THE VALVE DOES NOT APPEAR TO EXHIBIT THE COMMON GREEN COLOR INDICATING HEAT DAMAGE. DIFFERENTIAL COMPRESSION CHECK AND BORESCOPE INSPECTION PERFORMED IAW M-O MANUAL. UPON REMOVAL OF THE ROCKER BOX COVER TO FACILITATE REPLACEMENT OF THE CYLINDER, THE EXHAUST VALVE ROTOCOIL WAS FOUND TO BE SEVERELY DAMAGED.