As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER REMOVING AN ENGINE CYL TO REPLACE AN EXAUST STUD, IT WAS NOTICED THAT THE PISTON HAD CORROSION ON IT. ALL CYLINDERS WERE REMOVED AND PISTONS INSPECTED. CORROSION WAS FOUND ON ALL PISTONS. THE CYLINDERS WERE REPAIRED AND NEW PISTONS AND RINGS INSTALLED.