What was found
SUPPLEMENTAL INFORMATION (PLEASE REPLACE THE TEXT WITH THE FOLLOWING CORRECTED TEXT IN THIS FIELD): SERVICE BULLETIN 388C WAS PERFORMED IN CONJUNCTION WITH THE ANNUAL INSPECTION AND SERVICING OF THE AIRCRAFT. POST INSPECTION RUN-UP AND RETURN TO SERVICE FLIGHT WERE NORMAL. THE AIRCRAFT WAS RETURNED TO SERVICE. APPROXIMATELY 20 MINUTES INTO THE FIRST FLIGHT THE PILOT EXPERIENCED LOSS OF POWER AND ROUGH RUNNING ENGINE. VIBRATION WAS LESS WITH REDUCED POWER SO THE PILOT AUTOROTATED TO A SUCCESSFUL PRECAUTIONARY LANDING WITH NO DAMAGE TO THE AIRCRAFT. THE AIRCRAFT WAS TRANSPORTED BACK TO THE REPAIR STATION VIA FLATBED TRAILER FOR TROUBLESHOOTING AND REPAIR. A BORESCOPE INSPECTION REVEALED NUMBER 5 EXHAUST VALVE WAS NOT OPENING. THE VALVE COVER WAS NOTICEABLY DENTED FROM THE INSIDE NEAR THE EXHAUST VALVE ROCKER. FURTHER INVESTIGATION REVEALED THAT THE NUTS AND WASHERS SECURING P/N 72710 ROCKER SHAFT COVER HAD COME LOOSE AND FALLEN BETWEEN THE CYLINDERS ALONG WITH THE COVER. THIS ALLOWED THE ROCKER SHAFT TO BACK OUT OF POSITION ALLOWING THE ROCKER TO FALL FREE INSIDE THE ROCKER BOX. THE APPARENT CAUSE OF THIS SERVICE DIFFICULTY WAS DUE TO THE TECHNICIAN FAILING TO TIGHTEN THE ROCKER SHAFT COVER NUTS AFTER INSTALLATION AND THE INSPECTOR FAILING TO FULLY VERIFY PROPER INSTALLATION AND TORQUE OF HARDWARE ON ALL CYLINDERS. THE CYLINDER WAS REPAIRED AND RETURNED TO SERVICE