What was found
LEFT ENGINE ROUGHENED IN-FLIGHT, HAD TO BE SHUT DOWN. SAFE LANDING MADE. MAINTENANCE FOUND NR 4 CYL EXHAUST PUSH ROD PUSHED THROUGH VALVE COVER. FOUND ROCKER SHAFT BOLTS BROKEN. CAUSE OF FAILURE BELIEVED TO BE FROM UNDERTORQUE AND IMPROPER SETTING OF BOLT SAFETY LOCKS. CYLINDERS WERE INSTALLED 60 HOURS PREVIOUSLY. BOLTS ON OTHER CYLINDERS FOUND UNDERTORQUED AND SAFTIES NOT FULLY SET. CYLINDER SENT FOR TEAR DOWN. VALVES WERE REPLACED. C/W RE-TORQUE AND NEW SAFETY TABS ON ALL CYLINDERS. OIL FILTER INSPECTED. OIL CHANGED. PROPER USE OF A CALIBRATED TORQUE WRENCH AND NOT RE-USING SAFETY TABS MAY HAVE PREVENTED THIS INCIDENT.