What was found
PILOT REPORTED HE THOUGHT ALTERNATOR THREW THE BELT. REMOVED THE COWLING, FOUND ALTERNATOR LYING IN ENGINE COMPARTMENT, WIRING INTACT, COOLING FINS TORN OFF BELT PULLEY, BELT SHREDDD. AN7-40A MOUNTING BOLT NOT FOUND. BELT ADJUSTMENT BRACKET AN5 ATTACH BOLT FOUND IN COWLING WITH SAFETY WIRE, BOLT HOLE THREADS IN ENGINE CASE STRIPPED/WORN. INSTALLED HELI-COIL IN BOLT HOLE. INSTALLED EXCHANGE ALTERNATOR WITH NEW ATTACH HARDWARE AND BELT. RUN-UP CHECK NORMAL. PROBABLE CAUSE IS FAILURE OF AN7 BOLT OR NUT, ALLOWING ALTERNATOR TO BECOME UN-ATTACHED.