Incident Description
During flight, the pilot reported that the engine began missing and subsequently lost 10 inches of manifold pressure. In response, the pilot immediately set the mixture to full rich, switched fuel tanks, and activated the fuel boost pump, but the engine condition continued to worsen. Shortly thereafter, the oil pressure dropped to zero and the engine ceased operation. The pilot received radar vectors through clouds toward the nearest airport but determined that the aircraft could not glide that far. The airplane subsequently crash-landed in an open field.
Engine Examination
A teardown of the engine revealed that the #5 piston had sustained pre-ignition or detonation burn through of the ring belt structure between the lower spark plug and exhaust valve positions. The #5 connecting rod had overheated to the point of bearing destruction. Either the crankpin end of the rod or the release cap had ruptured the engine case.
Fuel System Settings
Examination of the fuel system showed that the fuel pump idle adjustment and aneroid had been set to high pressure (lean), and the fuel control needle valve had been adjusted to high flow. These settings would cause pre-ignition or detonation.
Probable Cause
The pilot's improper adjustment of the fuel system components, specifically setting the fuel pump idle adjustment and aneroid to high pressure (lean) and the fuel control needle valve to high flow, resulted in pre-ignition or detonation which led to engine failure.