What was found
AIRCRAFT ARRIVED AT FBO WITH RIGHT ALTERNATOR PROBLEM. REPAIRED RIGHT ALTERNATOR. WHEN AIRCRAFT START ATTEMPTED, LEFT STARTER FOUND TO HAVE SHEARED THROUGH-BOLT. NEW STARTER INSTALLED. AIRCRAFT DEPARTED FBO AND RETURNED ABOUT 30 MINUTES LATER WITH LEFT ENGINE SHUT DOWN AND FEATHERED. OPERATOR STATED RPM DROPPED IN-FLIGHT. ON GROUND CHECK, FOUND LEFT PROP SHAFT SHEARED FORWARD OF BULL GEAR. SUSPECT ENGINE HAD SEVERE VIBRATION IN RECENT HISTORY DUE TO DAMPENER COMING LOOSE CAUSING CRACK IN SHAFT. SUBMITTER RECOMMENDED ANY ENGINE SUBJECTED TO VIBRATIONS HAVE CRITICAL PARTS INSPECTED USING MAGNAFLUX/ZYGLO TECHNIQUES.