On June 16, 2003, at 0927 eastern daylight time, a Polstra Zodiac CH 601 HDS experimental airplane, N721PT, registered to and operated by the commercial pilot, collided with small trees and a tree stump during a forced landing in a field following a loss of engine power in Thomson, Georgia. The personal flight was operated under the provisions of Title 14 CFR Part 91 with no flight plan filed. Visual meteorological conditions prevailed. The commercial pilot was not injured, and the airplane sustained substantial damage. The flight departed Cobb County McCollum Field in Marietta, Georgia, at 0815 on June 16, 2003. The flight was en route to Thomson McDuffie County Airport, Thomson, Georgia. During cruise flight at 3,500 feet mean sea level approximately nine miles west of the airport, the engine began to run rough and lose power. The pilot stated the engine first made a tapping noise, followed by a louder noise. Attempts to restore engine power were unsuccessful. Approximately 6 miles from the airport, the propeller stopped completely and did not move during the pilot's attempts to restart the engine. The pilot executed an emergency landing in a field, and the airplane collided with small trees and a tree stump. Examination of the airplane revealed the nose gear was bent aft, the left main landing gear was bent, and the forward fuselage and wings were damaged. The propeller could not be turned by hand. Examination of the engine revealed the intake valve on the No. 3 cylinder was fractured in the transition region between the stem and the head. The pilot reported the engine was a Stratus EA-81 manufactured in 1997, and the total time on the engine was 83 hours since overhaul and 1.3 hours since inspection. The National Transportation Safety Board, Office of Research and Engineering, Materials Laboratory Division, performed metallurgical examination of the fractured valve head and stem portions, the spring seat, and half of the spring seat lock that were retrieved from the engine. Microscope examination of the mating fracture surfaces of the valve stem and head revealed the fine line features of the fracture were obliterated by mechanical damage. The valve stem portion displayed no bending deformation. The lock for the spring seat displayed fretting damage at the internal and external faces. Similar fretting damage was observed on the corresponding surface of the stem portion of the valve and on the corresponding bore face of the spring seat. The fretting damage was characteristic of higher than normal operating stresses or vibration and fatigue.