On June 14, 1998, approximately 1030 Pacific daylight time, a Cessna 172R, N9514P, recently purchased by Northwest Aircraft Rental (d/b/a Aurora Aviation), and being rented and flown by a private pilot, sustained substantial damage during a forced landing following a loss of power while in the traffic pattern at the Independence state airport, Independence, Oregon. The pilot and passenger were uninjured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and departed from Aurora, Oregon, earlier on the morning of the occurrence. The pilot reported that during the climb-out following a touch-and-go landing, the engine began to run rough, followed by smoke/oil spray issuing from the engine compartment. Shortly thereafter, he heard a loud bang and executed a forced landing in deep grass and vegetation. During the landing roll, the airplane nosed down, and the left wing was damaged. FAA on site examination revealed oil pooled beneath the engine and oil spray within the engine compartment. The aircraft was manufactured by Cessna Aircraft Company in 1998 and both it and the new Lycoming IO-360 engine had fewer than 190 flight hours. During the course of inspection of the engine after the accident, it was noted that there were signs of an external oil leak on the left side of the engine near the #4 cylinder, below the cylinder on the induction and exhaust pipes, on the side of the oil sump and on the left side of the lower engine cowling. A thumb-compression check revealed compression on all but the #4 cylinder. The #4 rocker cover was removed and valve train continuity was observed while rotating the crankshaft. A crack was observed in the steel barrel of the #4 cylinder, which appeared to be at least 180 degrees arc around the bottom half of the cylinder barrel. This crack appeared to be about 1/8" to 3/16" wide at the bottom of the cylinder. The engine was removed and shipped to the Lycoming factory in Williamsport, Pennsylvania, for further disassembly and examination. The #4 cylinder was removed for examination by the NTSB Materials Laboratory, and the remaining three cylinders were magnetic particle inspected (with no anomalies found), and then they were sectioned for further inspection (refer to Textron Lycoming Technical Report attached) which was reviewed by the NTSB Materials Laboratory staff). The NTSB Materials Laboratory examined the #4 cylinder and reported their findings (refer to attached report 98-149). Optical examination of the apparent fracture surfaces uncovered fracture markings, arrest lines and deformation patterns consistent with fatigue progression over the entire surface of the original crack. The portion that was fractured to open the barrel was consistent with the applied bending overstress forces. Ratchet marks and arrest lines indicated that the fatigue initiated at multiple sites on the exterior surface of the barrel wall in the groove between the 4th and 5th fins. The initiation region was located at the bottom of the cylinder between the 6:00 and 6:30 positions. Magnified viewing found four areas of multiple, clustered fatigue initiation sites on about 0.5 inch of the barrel outer diameter. Each cluster appeared to contain several individual fatigue origins that grew together as they penetrated the barrel wall. The origins were longitudinally located in the transition from the inboard radius of the #4 fin to the wall of the barrel. At the request of the NTSB Material Laboratory, Textron Lycoming prepared a metallographic specimen from a small section of the head side crack face and performed several tests and examinations at their laboratory. The testing included measurements of case depth, case hardness, core hardness and examinations of the sample's microstructure. Lycoming reported that the case depth, hardness and core hardness conformed to engineering drawing requirements and "no microstructural abnormalities" were found. Lycoming noted several "hairline" cracks up to 0.0004 inches depth in cross section. The engine and related parts were released to Textron Lycoming, as owner, on January 8, 1999. The airplane and records were released to HLM Air Services, Inc., at Independence, Oregon, on June 15, 1998.