On March 17, 1995, at 1520 hours Guam standard time, a Bell 47 helicopter, N6569H, crashed after a loss of engine power near Harmon Field, Guam. Visual meteorological conditions existed at the time of the local maintenance test flight. The helicopter sustained substantial damage and the two occupants were not injured. The investigation revealed that the engine lost oil pressure shortly after departure from Harmon Field. The pilot made a downwind landing into dense jungle. The tail boom separated from the aircraft during the crash sequence. The aircraft had a 100-hour inspection on March 16, 1995, and this was the first flight since the inspection. A Federal Aviation Administrator (FAA) inspector was on the island at the time on other business. This inspector interviewed the operator and the pilot a few days afterwards. The pilot was flying to an open field area at 400 feet above ground level when he noticed a change in the engine rpm and a loss of engine power. Due to obstacles, a 15 to 20 knot tailwind, and a loss of hydraulic power, the pilot was unable to make the open field and autorotated into the jungle. An examination of the aircraft, after it was removed from the accident site, revealed that the oil pressure relief valve was missing. According to the FAA inspector, the valve vibrated out within a short period of time after takeoff with a corresponding loss of engine oil. The operator told the FAA inspector that the valve was probably loose prior to takeoff. When asked if the valve was a normal preflight item, he stated that it should be at least checked to see if the safety wire securing the valve was broken. The operator does not have a published preflight procedures and/or preflight checklist nor is it a requirement under FAR Part 91 operations.