What was found
(CAN) IN CRUISE, NR 1 ENG MAGNETIC PLUG LIGHT CAME ON, PILOT ZAPPED IT OFF AND LIGHT WENT OUT. APPROX 5 MINUTES LATER, THE LIGHT CAME ON AGAIN AND COULD NOT BE ZAPPED, EXTINGUISHED. NR 1 ENGINE SHUT DOWN AND UNEVENTUAL LANDING CARRIED OUT. ONE SMALL SLIVER OF METAL ON PLUG. PLUG AND FILTER CLEANED, OIL SYSTEM REPLENISHED, GROUND RUN SERVICEABLE.