On August 14, 2017, about 1017 central daylight time, a Piper PA22-135 airplane, N2591A, was substantially damaged during a forced landing following a partial loss of engine power near Beckville, Texas. The pilot sustained minor injuries. The airplane was registered to and operated by private individuals as a Title 14 Code of Federal Regulations Part 91 flight. Day visual meteorological conditions prevailed. The flight was not operated on a flight plan. The pipeline surveillance flight originated from Cypress River Airport (24F), Jefferson, Texas, about 0800. The intended destination was the Gladewater Municipal Airport (07F), Gladewater, Texas.The pilot reported fueling the airplane at 24F before departing on his planned pipeline surveillance route. About 2 hours into the flight, en route to 07F, the engine began running rough. He initially established a course to Panola County Airport (4F2) about 8 miles away. However, the engine was not producing enough power to maintain altitude. He subsequently executed a forced landing to a pasture, which resulted in substantial damage to the left wing. A postaccident engine examination revealed that the no. 3 cylinder exhaust valve fractured at the stem seat. The intake valve appeared intact. The cylinder walls exhibited impact and scraping marks consistent with secondary damage from the separated exhaust valve head. The piston had fractured at several locations; the features appeared consistent with overstress fractures. The piston crown exhibited multiple impact marks similar to the cylinder walls. Metallurgical examination revealed that the fracture surface on the separated valve head was obliterated consistent with secondary mechanical damage after separation from the stem. The mating fracture surface on the stem was relatively undamaged. The fracture features were consistent with torsional loading at fracture. The stem was tapered adjacent to the fracture, and interconnected voids were observed at the fracture surface. Surface oxidation, sub-surface intergranular oxidation and deposits of lead-based compounds were also observed. The presence of the voids, the tapering of the stem, and the oxidation are consistent with high-temperature deformation under stress. Further examination revealed grain structure and sub-surface oxidation consistent with exposure to elevated temperatures over an extended period of time. The available airplane maintenance records included an entry, dated April 5, 1968, that noted: "major overhaul of engine this date;" the associated tachometer time was 1,153 hours. No subsequent entries regarding an engine overhaul were observed in the records. The most recent annual inspection was completed on April 22, 2017, at 1,770.4 hours tachometer time. The current owner purchased the airplane on April 23, 2017. The most recent engine maintenance consisted of an oil change on August 1, 2017. The tachometer indicated 1,986.5 hours at the accident site. The engine manufacturer recommended time between overhaul (TBO) was 1,500 hours operating time or 12 calendar years. Federal Aviation Administration regulations do not require Part 91 operators to comply with an engine manufacturer's TBO interval provided the engine meets annual inspection requirements.