What was found
PILOT NOTED VIBRATION IN LEFT ENGINE WHEN POWER REDUCED FOR LANDING. FOUND 3 OF 6 MOUNTING STUDS ON PROPELLER FRACTURED. REMOVAL OF PROPELLER REVEALED O-RING PINCHED BETWEEN PROPELLER HUB AND STARTER RING GEAR. PROPER LOCATION OF O-RING IS INSIDE PROPELLER FLANGE. AS O-RING EXTRUDED OUT, BOLT TORQUE REDUCED ALLOWING PROPELLER TO WORK AND MOVE FRACTURING BOLTS. PROPELLER HAD RECENTLY BEEN REMOVED AND REPLACED FOR SPINNER ASSEMBLY REPLACEMENT. FURTHER INVESTIGATION REVEALED STARTER RING GEAR CRACKED AND HUB CRACKED AT ONE MOUNTING STUD HOLE. RING GEAR AND HUB REPLACED. SUSPECT CAUSE: FAILURE TO FOLLOW MANUFACTURER'S INSTALLATION INSTRUCTIONS.