What was found
(CAN) ENGINE PLACED ON (PISTON ENGINE ON CONDITION PROGRAM). DURING ON-CONDITION 100 HOUR INSPECTION, THE NR 2 CYLINDER COMPRESSION TEST WAS LOW AND LEAKING FROM VALVES. ON FURTHER INSPECTION, EXHAUST VALVE STEM WAS FOUND TO BE ERODED TO ABOUT HALF OF DIAMETER OF STEM. THE OTHER 3 CYLINDERS WERE INSPECTED AND FOUND EXHAUST VALVE STEMS TO BE ERODED AS WELL. OPERATOR OF THIS AIRCRAFT, HAS A STANDARD PRACTICE TO LEAN THE AIRCRAFT DURING TAXI AND IN FLIGHT. THIS MAY HAVE LEAD TO THIS FAULT FROM IMPROPER LEANING PROCEDURES FROM STUDENT PILOT'S. INSTRUCTORS HAVE BEEN MADE AWARE OF PROPER LEANING PROCEDURES. THE ENGINE WAS OPERATED 282.5 HOURS BEYOND TBO. AIRCRAFT WAS REMOVED FROM SERVICE.