The aircraft was cruising at 9500 feet when the oil filter adaptor became detached from the engine, causing complete loss of oil pressure. As a result the number two big end failed and the number two connecting rod punctured the crankcase. During the ensuing forced landing, on a road, the pilot was unable to fully manually lower the landing gear. After touchdown the right main gear leg collapsed, causing the aircraft to strike a dirt bank. The oil filter adaptor thread had been damaged by overtightening and this allowed the adaptor to detach from the housing on the oil pump casing. Following the failure of the engine it became necessary to lower the landing gear by using the emergency system. Hydraulic power to lower the gear is normally provided by the engine driven hydraulic pump. It was determined that the pilot was unable to fully extend the gear with the emergency gear extension handle as the handle had apparently inadvertently been moved slightly inward. Any inward movement of the handle jams the pump action making the handle difficult to move thus giving the impression that the gear is fully extended.