On July 19, 1998, about 2035 eastern daylight time, a Cessna 152, N69193, was substantially damaged during a forced landing near Philippi, West Virginia. The certificated flight instructor and student pilot were not injured. Visual meteorological conditions prevailed for the dual instruction flight. No flight plan had been filed for the local flight which originated from Benedum Field, Clarksburg, West Virginia, about 1945, and was conducted under 14 CFR Part 91. In the NTSB Form 6120.1/2, Pilot/operator Aircraft Accident Report, the flight instructor reported that the engine lost power and dropped to 1,300 RPM. The airplane was unable to maintain altitude, and a forced landing was performed on an uneven field, during which the nose landing gear collapsed. An inspector from the Federal Aviation Administration (FAA), reported that the airplane touched down on the main landing gear, on the level portion of a field. It then encountered rising terrain, where the nose wheel dug into the ground, collapsed rearward, and punctured the lower fuselage. The left main landing gear was twisted in the airplane and skin wrinkles were observed above the fuselage attach point. In addition, the engine mounts were bent and the firewall was wrinkled. The airplane was equipped with a Lycoming O-235-L2C engine. The FAA Inspector reported that examination of the engine revealed the number 2 exhaust valve was stuck in the valve guide, and the valve head had separated from the valve stem. The valve guide with valve stem stuck in it was loose in the engine and fell out of the engine when the valve cover was removed. The portion of the valve guide that was exposed in the rocker arm area was covered with a carbon like substance similar to coking. Portions of the valve head were recovered from the exhaust and intake manifolds The number 2 cylinder had been installed on October 21, 1997, 172.3 hours prior to the accident. The numbers 1, 3, and 4 cylinders were installed on March 2, 1998, 51 hours prior to the accident. The chief inspector for KCI Aviation reported that he examined the other intake and exhaust valve guides from the rocker arm side of the cylinders, and none had any coking visible on them.