What happened
During flight, the pilot noticed a decrease in oil pressure and elected to land at an alternate airport. While en route, a complete loss of oil pressure occurred and the pilot executed an emergency landing in an open field. While on the landing roll, the aircraft hit a soft spot and nosed over.
The investigation
An examination of the engine revealed there was a dimple and hole in the rocker box cover of the No 3 cylinder. A loose 3/8-24 hexagon jam nut (for the intake rocker arm adjusting screw) was found inside the rocker box cover. The hole and dimple in the rocker box cover were opposite of the exhaust rocker arm adjusting screw and matched the size and shape of the loose nut.
Findings
The loss of oil pressure was caused by a loose hexagon jam nut that had become dislodged from the intake rocker arm adjusting screw. The nut traveled through the engine, creating a hole in the rocker box cover of the No 3 cylinder. The aircraft nosed over upon landing due to hitting a soft spot on the ground.