As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INSTALLED NEW PLUGS IN ENGINE. PERFORMED RUN-UP. OPS OK. FOUR HOURS LATER, ENGINE HAD A 600 RPM DROP. REMOVED SPARK PLUGS, FOUND A PIECE OF WIRE AROUND CENTER ELECTRODE OF 3 PLUGS AND 4 OTHER PLUGS NOT FIRING VERY HOT ON PLUG TESTER. OPENED ANOTHER SET OF LUGS AND FOUND PIECE OF METAL RING AROUND CENTER ELECTRODE. APPEARED TO BE FROM MANUFACTURING PROCESS. LOT NR 4G99.