On July 20, 2002, approximately 1030 Pacific daylight time, a Cessna 150J airplane, N51286, operated by a private owner (d/b/a Dave's Aircraft of Apex Airpark, Silverdale, Washington) and being flown by a private pilot who had rented the aircraft, was substantially damaged in a forced landing in high vegetation following a reported loss of engine power near Pe Ell, Washington. The pilot and one passenger received serious injuries in the accident. Visual meteorological conditions prevailed and no flight plan had been filed for the 14 CFR part 91 personal flight, which departed Auburn, Washington, about 0920. In an initial telephone interview and subsequent written statement, the pilot reported that his original destination was Seaside, Oregon, but that upon arrival in the Seaside area, fog and clouds precluded his landing there. He reported that he then decided to turn back north and go back for breakfast at Chehalis, Washington, or Olympia, Washington. The pilot reported that about 10 minutes north of the Columbia River, the engine started running rough and would only develop about 2,300 RPM. The pilot stated that he tried using carburetor heat for about 90 seconds, without effect. The pilot stated that he then started seeing a little bit of smoke or oil coming from the engine area. He reported that he then heard a "bang", saw something depart the engine area, and the engine lost all power. He then performed a power-off landing in an old grown-over clear-cut area, and as the aircraft touched down it became entangled in the high vegetation and nosed over. The pilot stated that after landing he called for assistance using a cell phone. The aircraft was eventually located by a search aircraft, and both occupants were rescued by US Army Forces. The engine was later removed and examined. The #2 cylinder barrel was found to be fractured completely around the circumference of the barrel and separated more then 2 inches exposing the damaged piston and connecting rod. The fracture was between cooling fins and was located approximately 1/3 of the distance below the head/barrel intersection and the cylinder hold-down flange. The separation of the barrel had distorted the engine baffles and allowed the induction pipe to become detached from the manifold. The cylinder, a new standard size Superior Airparts SA 10200 (FAA PMA), had been installed during an engine major overhaul that was completed on March 2, 1998. Since that time, the cylinder had accumulated approximately 1,100 hours time in service. It was examined by the NTSB materials laboratory, and it was determined that the failure was the result of the propagation of a fatigue crack that initiated in the root area between the eighth and ninth cooling fins (see attached materials laboratory factual report). The crack advanced circumferentially until the cylinder wall failed between the eighth and tenth fins. Further examination of the initiation site revealed no evidence of surface defects, such as abusive machine marks, scratches, or corrosion pits. In addition, it was determined that the origin area did not contain any material defects, and the average hardness of the barrel material was HRC 28, which confirms to the drawing requirements of HCR 28 to 34. The root radius of the fins through which the facture propagated met the minimum 0.062 inch drawing requirement, and the barrel wall thickness met the minimum 0.078 inch minimum thickness. The barrel bore diameter varied between 4.060 and 4.062 inches, with the requirement being 4.062 to 4.064 inches. The microstructure of the cylinder walls was consistent with through-hardened quenched and tempered steel, and there was no evidence that the cylinder had been ground oversize. An energy dispersive spectroscopy (EDS) analysis determined that the base metal contained a composition consistent with 4140 steel as required by the drawings. There was no evidence of inadequate lubrication or overheating, and no root cause for the initiation of the fatigue fracture could be determined.