What was found
(CAN) CREW EXPERIENCED ROUGH RUNNING ENGINE (RT) WHILE ON APPROACH. CREW POWERED BACK, LANDING WITHOUT FURTHER INCIDENT. UPON INVESTIGATION BY MAINTENANCE, IT WAS DISCOVERED THAT 2 NUTS HAD BACKED OFF ON THE COVER PLATE FOR THE ROCKER PIN CAUSING THE PLATE TO FALL OFF. THIS CAUSED THE PIN TO MOVE ITS WAY OUT OF THE CYLINDER CAUSING THE PUSHROD TO BEND AND SUBSEQUENT OIL LEAKAGE. CYLINDER ASSEMBLY WAS REPLACED. AIRCRAFT RELEASED TO SERVICE WITHOUT FURTHER INCIDENT.