On July 1, 2000, at 1300 mountain daylight time, a Diamond Aircraft Industries 20C-1, N991CT, sustained minor damage when the propeller separated from the airplane near Spanish Fork, Utah. The private pilot and his passenger were not injured. The airplane was being operated by Smith and Barlow Enterprises, Inc., Provo, Utah, under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the local flight that originated from Provo, Utah, approximately 15 minutes before the incident. No flight plan had been filed. The pilot said that he had just departed on a cross-country flight, and had climbed to 9,500 feet. He said that he began to feel a "heavy vibration" in the aircraft, and immediately turned the airplane back towards the closest runway at Spanish Fork Airport. Soon thereafter the propeller separated from the airplane. The pilot performed a forced landing to runway 30 without further incident. The wooden propeller was not recovered. The propeller extension assembly was removed from the engine and forwarded to the National Transportation Safety Board's (NTSB) Material's Laboratory. Examination revealed that all six propeller attachment bolts showed signs of fatigue failure. There was also fretting damage on the propeller extension flange and bushing shanks. Postincident examination of the maintenance records indicated that the attachment bolts for the propeller were last torqued on April 26, 2000, 110 hours before the incident. Sensenich Propeller Manufacturing Co., Inc., the manufacturer of the propeller, recommends that their wooden propellers be torque checked every 50 hours of flight.