Incident Description
A commercial pilot with 15,000 hours of flight experience was conducting an IFR cross-country flight at 12,000 feet mean sea level (msl) when he detected smoke and noticed the engine began to surge. He requested a lower altitude, and the air traffic controller provided the location of the nearest airport. During the descent, at approximately 9,000 feet msl, the engine lost oil pressure. The pilot shut down the engine using the mixture control. The engine continued to turn over, and the pilot heard several loud explosions and observed fireballs at the propeller hub before the propeller stopped rotating.
Unable to reach the intended airport, the pilot elected to land in a field located about half a mile south of the airport. He maneuvered the airplane under a set of power lines and landed in an open field.
Engine Examination
A detailed examination of the engine was performed at the engine manufacturer's facilities. The crankshaft and counterweights assembly exhibited lubrication distress of varying degrees on all connecting rod journals and bearings. The number six piston head showed evidence of detonation and pre-ignition damage.
Conclusion
The examination concluded that a turbo-normalizing system had been installed on the engine. The engine was originally certificated as a normally aspirated model and contained high compression pistons. According to the examination, the installation of a turbo-normalizing system on such an engine is contrary to the recommendations in TCM Service Bulletin M64-18 and can reduce the engine's detonation margins.