On August 15, 1998, approximately 1620 mountain daylight time, a Cessna 175, N8512X, nosed over during a power-off forced landing about 15 miles southwest of Missoula, Montana. The private pilot and his passenger were not injured, but the aircraft, which was owned and operated by Hinkle Aviation, of Caldwell, Idaho, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which departed Kalispell, Montana, about 50 minutes prior to the accident, was being operated in visual meteorological conditions. No flight plan had been filed, and there was no report of an ELT activation. According to the pilot, while at cruise, smoke started accumulating in the cabin, so he started checking for abnormal instrument indications. Soon thereafter, he noticed that the oil pressure had dropped to zero. He then reversed course in order to return to an open field he had flown over a few minutes earlier. As he arrived over the field, the engine stopped running, and he attempted a forced landing. His touchdown was successful, but as the aircraft neared the edge of the field, the nose gear hit a small pine tree. When the gear hit the tree, the entire nose gear assembly was torn from the fuselage and the aircraft nosed over. A post accident inspection of the engine revealed that the oil filler tube had separated from the engine and was not found at the accident site. Almost all of the engine oil had drained from the crankcase, and the number three connecting rod had failed and come through the side of the case.