As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING A CYLINDER CHANGE ON THE ENGINE THE EXHAUST RISERS WERE REMOVED TO ALLOW FOR INSTALLATION. WHILE CHECKING THE EXHAUST PORTS FOR FOD IT WAS NOTICED THAT THERE WAS ABNORMAL WEAR ON THE NR 3 EXHAUST VALVE JUST ABOVE THE MUSHROOM. FURTHER INSPECTION FOUND THAT THE EXHAUST VALVE ON THE NR 6 CYLINDER HAD THE SAME PROBLEM.