On August 13, 2005, about 1030 mountain daylight time, a Piper PA-24-250 Comanche, N6677P, sustained substantial damage following a loss of engine power and off airport forced landing near the Challis Airport (LLJ), Challis, Idaho. The airplane is owned by the pilot and was being operated as a visual flight rules (VFR) personal/pleasure flight under the provisions of 14, CFR Part 91, when the accident occurred. The private pilot and passenger received serious injuries. Visual meteorological conditions prevailed, and no flight plan was filed for the flight that originated at Challis approximately 3 minutes prior to the accident. The pilot's intended destination was Mackay, Idaho. During a telephone conversation with the National Transportation Safety Board IIC on August 15, the pilot reported that shortly after takeoff from runway 34, the propeller RPM became erratic and uncontrollable and he elected to return to Challis. He reported that during the descent into Challis the airplane developed a rough running engine and eventually lost power. The pilot stated that he landed in a hay field, short of runway (16), resulting in substantial damage. Personnel at the Challis airport reported that a large puddle of oil was found in the run-up area following the airplane's pre takeoff run-up and departure. The airplane was equipped with a Lycoming O-540-A1C5 engine. Examination of the engine's maintenance records revealed that the engine had been overhauled and inspected on July 13, 2004, subsequent to a "prop strike." The records indicated that the "tach time" at overhaul was 255.6 hours, approximately 35.2 hours prior to the accident. Additional maintenance records for this accident can be found in the public docket. On August 25, representatives from the Federal Aviation Administrations Boise, Idaho, Flight Standards District Office, (FSDO), Textron Lycoming and The New Piper Aircraft Company performed an engine examination and teardown. Preliminary external examination of the assembly revealed a large hole in the engines crankcase, adjacent to the number four cylinder assembly. The number four piston rod was fractured and extensive heat distress, deformation and discoloration was noted to both the number three and number four piston assemblies. The representative from Textron Lycoming reported that this type of damage is indicative of oil depravation. Further examination of the engine assembly revealed that the hydraulic pump adapter pad cover gasket was protruding from between the accessory mounting pad and its associated cover (part number 89106). A section of the gasket (part number 68315) separated and was displaced from its installed position. The FAA Airworthiness Inspector overseeing the examination reported that the installed gasket (Part number 68315) was intended to be used in conjunction with a hydraulic pump, if applicable, however, this particular engine configuration does not utilize a hydraulic pump, and therefore a hydraulic pump adapter pad gasket (part number 69551) should have been installed. The Lycoming O-540-A series Parts Catalog illustrates the use of gasket number 69551 when utilizing the hydraulic pump adapter pad cover.