On May 23, 1993, at approximately 1115 hours Pacific daylight time (PDT), a Cessna C-185A, N4080Y, registered to and operated by Clarence R. Forbes, and being flown by Howard S. Hash, a certificated commercial pilot, was substantially damaged when the aircraft nosed over during a precautionary landing in a boggy field following an emergency descent due to a partial loss of power. The landing took place on the Idaho side of Lolo Pass at the Idaho/Montana border and approximately 50 nautical miles northeast of Lowell, Idaho. The pilot was uninjured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which was personal in nature, was to have been operated in accordance with the requirements set forth in 14CFR91 and departed from Stevensville, Montana, earlier in the morning intending to return to the same airport. The pilot reported in a telephonic interview with the investigator, that he was cruising at 8000 feet above mean sea level (MSL) when a partial power loss occurred. He stated that he was familiar with the terrain and, upon seeing oil on the left wing strut, became concerned that the engine might seize. He then executed a precautionary landing in a meadow. The pilot reported that the meadow was boggy and the aircraft nosed over during the rollout. Post crash examination of the engine by the pilot revealed a hole in the number two cylinder rocker box cover (refer to photograph 1). Post crash disassembly of the engine revealed that the number two cylinder exhaust rocker arm was broken in half through the center pivot bearing area (refer to photograph 2). The rocker arm halves and rocker box cover were shipped to the Safety Board's Materials Laboratory for metallurgical examination. The examination revealed three areas of fatigue progression on the surfaces of the rocker arm halves, two of which originated in the drilled oiling hole (refer to attached Metallurgist's Factual Report). The number two cylinder exhaust rocker arm, one of 12 such rocker arms, was provided by Clyde G. Ritter, for installation on N4080Y. The rocker arms were installed approximately 11 hours previous to the accident, during a major overhaul of the engine. A new camshaft provided by Mr. Ritter was installed at the same time, however, the Supplemental type Certificate for the camshaft (STC# SE5784NM) was not applicable for any IO-470 series engines. Additionally, no STC has been issued for the rocker arms. N4080Y did not have an experimental certification at the time of the accident.