THE ACCIDENT AIRPLANE WAS IN CRUISE FLIGHT WHEN THE PILOT NOTED AN ENGINE VIBRATION FOLLOWED BY AN INFLIGHT SEPARATION OF THE PROPELLER AND SPINNER. THE PILOT DECLARED AN EMERGENCY AND LANDED IN A SOYBEAN FIELD. DURING THE LANDING ROLL THE AIRPLANE NOSED OVER IN SOFT TERRAIN. THE PROPELLER AND SPINNER WERE RECOVERED STILL ATTACHED TO THE FRONT PORTION OF THE ENGINE CRANKSHAFT. EXAMINATION OF THE CRANKSHAFT SHOWED AN AREA OF WEAR AT THE KEYSLOT WHERE THE PROPELLER FLANGE IS FITTED OVER THE SHAFT. EXAMINATION OF THE INSIDE WALLS OF THE PROPELLER FLANGE SHOWED AN 'AREA OF CONCENTRATED FRETTING WHICH LEFT A DISTINCT WEAR MARK THAT MATCHES THE FRACTURE SURFACE OF THE FAILED CRANKSHAFT.' ACCORDING TO MAINTENANCE RECORDS THE CONVERTED AUTOMOTIVE ENGINE HAD BEEN IN SERVICE SINCE MAY 23, 1987 AND HAD ACCUMULATED APPROXIMATELY 387 HOURS IN SERVICE. THE LAST ANNUAL INSPECTION ON THE AIRPLANE WAS ACCOMPLISHED ON MAY 21, 1992, APPROXIMATELY 3 FLIGHT HOURS BEFORE THE ACCIDENT.