What was found
PROPELLER REMOVED TO INSTALL NEW ALTERNATOR BELT. FOUND SPINNER BACK PLATE CRACKED 320 DEGREES AROUND MOUNTING HOLES, COULD NOT BE SEEN UNLESS PROPELLER AND SPACER P/N B4381 REMOVED. SUBMITTER STATED THIS IS THIRD SPINNER BACK PLATE FOUND CRACKED IN SAME AREA. OTHER 2 WERE C150 AIRCRAFT. RECOMMEND AD BE ISSUED FOR REMOVAL AND INSPECTION OF BACK PLATE BY VISUAL, DYE PENETRANT INSPECTION. SUBMITTER STATED LOSS OF BACK PLATE IN-FLIGHT COULD CAUSE LOSS OF SPINNER AND POSSIBLE DAMAGE OR LOSS OF AIRCRAFT.