On May 23, 2007, about 1655 eastern daylight time, a Cessna 172E, N3982S, was substantially damaged during a forced landing, shortly after takeoff from Leesburg Municipal Airport (JYO), Leesburg, Virginia. The certificated private pilot received minor injuries. Visual meteorological conditions prevailed and a visual flight rules (VFR) air defense identification zone (ADIZ) flight plan was filed for the flight destined for Culpeper Regional Airport (CJR), Culpeper, Virginia. The personal flight was conducted under 14 Code of Federal Regulations Part 91. According to the pilot, the purpose of the flight was to relocate the airplane to CJR for the airplane’s annual maintenance inspection. After departure from runway 17, the pilot started a turn to the west and made his initial call to air traffic control (ATC). After the airplane crossed state route 15, the pilot made a power reduction to 2,400 rpm. The engine "without warning lost most of its power" and began to "shake and misfire violently." The pilot attempted to control the engine by adjusting the throttle and mixture controls, without result. He then declared an emergency with ATC and began to receive vectors from the controller. He looked for a place to land and spotted an open field. He obtained best glide speed, turned on to a base leg for the field he had selected, and after turning on to a final leg, extended his wing flaps to the 40-degree position and touched down. The airplane passed through a row of trees located halfway down the field; however, a ditch existed on the opposite side. The pilot attempted to get over the ditch by raising the flaps, but the airplane struck the far side of the ditch, the nose gear collapsed and the airplane was substantially damaged. The pilot held a private pilot certificate with a rating for airplane single-engine land. He had accumulated 235.7 total hours of flight experience; 53.1 hours of which were in the accident airplane make and model. According to Federal Aviation Administration and maintenance records, the airplane was manufactured in 1964. The airplane’s most recent annual inspection was completed on May 18, 2006, and at the time of the accident, it had accumulated 4017.3 total hours of operation. According to the pilot, the engine had accumulated 2,996.5 total hours of operation at the time of the accident. An overhaul of the engine had been completed on March 25, 2005. The time since overhaul was 34.8 hours and the engine had been operated 6.9 hours since the most recent annual inspection. Examination of the airplane and engine by National Transportation Safety Board investigators revealed no preimpact malfunctions of the airplane or its systems; however, during examination of the engine, it was discovered that the No.2 cylinder’s exhaust valve rocker arm was fractured and had separated into two parts. The fractured exhaust valve rocker arm, rocker shaft, push rod, and intact intake valve rocker arm were examined by the Safety Board’s Materials Laboratory. The examination revealed that the exhaust valve rocker arm had fractured at two locations through the rocker shaft hole, with about one-third of the pivot hole circumference on one piece of the arm and two-thirds on the other. Both fractures intersected oil holes on the inside diameter of the rocker shaft hole. The features on the push rod side fracture emanated from a single location at the forging flash line on the inboard side of the arm. This fracture propagated into the arm, turning into the inside diameter of the arm’s central pivot hole. The fractographic features were rough, and when viewed optically, were indicative of overstress. No crack arrest positions were noted. Examination of this fracture with a scanning electron microscope did not revealed any features conclusively indicative of other cracking modes. Optical examination of the fracture on the valve side of the rocker arm revealed that more than half of the fracture contained features that were typical of fatigue cracking. Fatigue initiation was from the pivot hole inner surface adjacent to the oil hole. Fatigue propagation extended approximately 5 millimeters from the pivot hole inner surface into the arm. Fracture features beyond that depth were typical of overstress. Compression damage on the portion of the fracture furthest from the origin area, and scuffing damage on the bushing adjacent to this fracture, were consistent with the exhaust valve side fracture being secondary to the fracture on the push rod side of the arm. The wear pattern on the exhaust valve contact surface on the exhaust valve rocker arm was off-center and blurred. Two overlapping wear patterns were noted on the intake valve contact surface on the intact intake valve rocker arm. No unusual wear patterns were noted on the push rod contact area on either of the rocker shafts. No damage or unusual wear patterns were noted on the rocker shaft. The push rod however, exhibited a bend measuring approximately 0.005 inch. According to the owner, one of the cylinders was purchased on "eBay." During examination of the accident airplane’s engine maintenance records, no serviceable tags for the engine cylinders were discovered. Further examination of the engine maintenance records revealed that the engine removed previously was listed as serial No. 33130-D-5-D. The engine listed as having been installed after overhaul was serial No. 33150-D-5-D. Examination of the engine revealed that the data plate on the engine was flat, did not conform to the shape of the case, was held on with two rivets, and displayed the serial number as 33130-D-5-D.