HISTORY OF FLIGHT On July 11, 1998, at 1247 central daylight time, a Cessna 172N airplane, N5632E, owned and operated by W. J. Bonefas Co., Inc., of Manvel, Texas, was destroyed during a forced landing, following a loss of engine power near Lincoln, Texas. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations (CFR) Part 91 personal cross-country flight. The private pilot and the two passengers were seriously injured. The flight originated from the Wolfe Air Park Airport, Manvel, Texas, earlier in the day and proceeded to the Lampasas Airport near Lampasas, Texas, to drop off a passenger. The aircraft was fueled with 32.7 gallons of 100 octane low lead aviation fuel prior to departing the Lampasas Airport. The aircraft departed Lampasas sometime between 1100 and 1130. Witnesses observed the airplane maneuvering at a "low" altitude with black smoke coming from the engine. During landing toward the southwest on State Highway 21, at the intersection of Farm Road 1624, the airplane went under the power lines which span State Highway 21 striking the lower wire, and continued under the flashing traffic signal lights which also span State Highway 21. The airplane's right wing tip impacted a stop sign. The airplane touched down and bounced, veered to the right, and then impacted the Walch Store's gasoline pumps. The airplane exploded into flames and was consumed by the post crash fire. PERSONNEL INFORMATION The pilot's flight logbook was not located, therefore, his last biennial flight review and total flight time could not be determined. A few burnt pages of a pilot flight logbook were found within the area of the airplane's cockpit. FAA records indicate the pilot reported having accrued 105 total flight hours on his application for a class three medical examination, dated July 1, 1998. The pilot was issued a Private Pilot certificate on July 17, 1992. AIRCRAFT INFORMATION The 1978 Cessna 172N was a single engine, four-place airplane. The airplane was equipped with a fixed pitch propeller and a Lycoming O-320-H2AD engine, which produces 160 horsepower. An estimate of the weight of the airplane at the time of the accident placed it within weight and balance limits. According to the operator, he could not locate the airplane's airframe and engine logbooks. WRECKAGE AND IMPACT INFORMATION Examination of the accident site revealed that the aircraft came to rest upright on a 200 degree measured magnetic heading at the Walch Store's gasoline pump island, about 205 feet from the power lines which span State Highway 21. The aircraft was consumed by fire from the firewall back to the leading edge of the vertical stabilizer. Continuity was confirmed to all flight control surfaces. An on-site examination of the engine revealed that the #3 cylinder exhaust valve rocker arm fulcrum was displaced from the cylinder at the rocker arm retaining stud. The cylinder was removed from the engine and retained by the NTSB IIC for further examination. TEST AND RESEARCH On July 27, 1998, under the supervision of the NTSB IIC, the engine and the #3 cylinder were disassembled and examined at Air Salvage of Dallas, Lancaster, Texas. The examination of the #3 cylinder revealed that the gap between the exhaust valve rocker arm fulcrum and the cylinder was approximately 3/8 inch. The exhaust valve rocker arm retaining nut was attached to the cylinder stud; however, the rocker arm retaining stud could be turned by hand. The exhaust valve cap was found loose in the rocker box, and the rocker box cover was found to have been weld repaired in the area of the intake rocker arm. According to the engine manufacturer's representative, "the shroud tube lockplate appeared to be recently installed as it was not varnish coated like the lockplates in the other cylinders. This may indicate a recent maintenance activity on the #3 cylinder." See the engine manufacturer's teardown report for further details of the examination. The #3 cylinder was shipped to Textron Lycoming, Williamsport, Pennsylvania, for further examination. On September 16, 1998, under the supervision of an FAA inspector, the cylinder was examined. According to the manufacturer, "the exhaust rocker arm retaining stud (.3125-18 NAT'L FORM) conformed to blueprint specifications. The exhaust rocker arm retaining stud-tapped hole in the cylinder head did not conform to blueprint specifications. As per the blueprint, the stud-tapped hole is required to be drilled .259/.266 to a depth of 1.19 inches. The tap is 3100-18 NC-3 with a P.D. (pitch diameter) of .2739/.2769 SPL. The P.D. of this tap is not the P.D. of a standard tap; it is undersize in order to acquire the proper driving torque of the stud. The existing threaded hole was found to be well over the maximum P.D. as specified on the blueprint. The largest thread gage available was .2803 and [it] was very loose in the hole." See the engine manufacturer's report for further details of the examination. ADDITIONAL DATA The airplane wreckage was released to the owners representative.