On June 20, 1993, about 1400 hours Pacific daylight time, a homebuilt experimental Sill/Thomas Long EZ airplane, N977ST, was substantially damaged during an emergency landing on Owens Dry Lake in Mono County, California. Visual meteorological conditions prevailed for the personal cross country flight and a VFR flight plan was filed. The pilot received minor injuries. The flight had originated at Mammoth Lakes, California, at 1330 hours and was destined for Van Nuys, California. The pilot stated that he was in level cruise at 10,500 feet MSL when the engine suddenly quit. He stated that he declared an emergency and set his transponder to 7700. He made his emergency landing on Owens Dry Lake, which was soft from recent rains. The nose gear collapsed and the airplane nosed over inverted. The pilot said that the emergency locator beacon (ELT) transmitted for about three hours. At 1835, a Forestry Service helicopter responding to the ELT signal rescued the pilot from the inverted wreckage. Post crash examination of the engine revealed that the exhaust valve head in the number four cylinder was found separated from the valve stem. Examination of the number four cylinder head valve spring area revealed heavy oil coking and oxidization with carbon deposits evident. Numerous airworthiness discrepancies were noted during the examination of the engine and are documented in photographs attached to this report. The valve parts from the number four cylinder were sent to the Safety Board laboratory in Washington, D.C., for detailed metallurgical examination. The complete report of the component examination is attached to this report. In summary, the metallurgical report noted that the valve stem in the area of the fracture was; 1) devoid of the required chrome plate, 2) reduced in diameter, and, 3) reduced in hardness. AIRCRAFT INFORMATION Review of the maintenance records revealed that the aircraft was assembled between October of 1982 and June of 1986. An Experimental airworthiness certificate in the amateur built category was issued for the aircraft on February 7, 1986. The letter of Operating Limitations issued by the FAA at the time the airworthiness certificate was executed require that a condition inspection (Annual Inspection equivalent) be accomplished each 12 months in accordance with 14 CFR 43, appendix D. Subsequent to the airworthiness certificate issuance entry in the aircraft logbooks, no condition inspection entry was found. According to factory records and the engine logbook, the engine was manufactured by Lycoming and shipped to a distributor on February 3, 1984. Lycoming records and log book information indicate that the engine was installed in the accident aircraft on June 2, 1985. At the time of the accident, the engine had accumulated 484 hours of operation.