On October 9, 1999, at 1500 central daylight time, a Cessna 150G airplane, N3603J, sustained substantial damage during the forced landing following a loss of engine power near El Reno, Oklahoma. The airplane, owned by Timeless Images, Inc., of Edmond, Oklahoma, was operated by Sunshine Aviation, of Guthrie, Oklahoma, under 14 Code of Federal Regulations Part 91. The commercial pilot/flight instructor received minor injuries, and the student pilot was not injured. Visual meteorological conditions prevailed for the cross-country instructional flight, and a flight plan was not filed. The flight departed Guthrie, Oklahoma, at 1430, with a planned destination of Hobart, Oklahoma. Local authorities, the operator and the pilots reported to the FAA inspectors that a total loss of engine power occurred during cruise flight at 3,500 feet msl, approximately 6 miles west of the El Reno Municipal Airport. The flight diverted toward the airport; however, the airplane landed approximately 3 miles west of the El Reno Municipal Airport. During the landing flare/touchdown in the soft field, the nose landing gear collapsed and the aircraft spun around. The wings and empennage received structural damage. The nose landing gear and propeller were damaged. On October 20, 1999, the FAA inspector and the manufacturer representative examined the Continental O-200-A engine, serial number 64599-6-A. During the examination, the #4 cylinder intake rocker arm was found separated at the point where it attaches to the valve rocker shaft. The #4 cylinder components were forwarded to the NTSB Materials Laboratory for metallurgical examination. The metallurgical examination of the rocker arm indicated that a fatigue fracture initiated on the push rod side of the rocker arm. Optical and SEM examination of the surface of the rocker arm in the vicinity of the origin area revealed no evidence of machining defects or mechanical deformation associated with the origin. A portion of the contact surface of the valve side of the rocker arm had a smooth, metallic appearance, indicating off-center contact with the valve stem. See the enclosed metallurgist's report for additional details. The FAA inspector reported that a new #4 cylinder was installed on November 13, 1998. The FAA inspector reported the cylinder had been manufactured by [Aircraft] Engine Components, Inc., at San Antonio, Texas. This cylinder had accumulated 76 hours at the time of the rocker arm failure. There were no maintenance entries for repair, overhaul, or replacement of the rocker arm. Total time on the engine was 3,198 hours. See the enclosed maintenance records for additional information. The NTSB IIC examined the #4 cylinder components in March 2000 at Arlington, Texas. Vibro peened on the cylinder were the following numbers: head part number AEC65315D; casting number 67333-22; and PMA part number AEC65314-D FAA-PMA. According to personnel at Engine Components, Inc., the FAA Form 8130-3 with work order number 01118354 was for the #4 cylinder (part number AEC65314-A, serial number 67333-22H).