The propeller from a small private airplane came off in flight. The head ends of all six attachment bolts remained in the propeller hub when it was found. Two threaded shanks with nuts remained with the engine, while the remaining four shank ends with their nuts were missing. Parts available for examination, in addition to the hub and attachment bolts, were the two propeller blades and the engine crankshaft. The purpose of this examination was to determine the nature and probable cause of failure in the six attachment bolts. Indications of fatigue failure and wear were the major findings...