On December 29, 2004, approximately 1735 central standard time, a single-engine Beech A36 airplane, N920GL, sustained minor damage during a forced landing following a partial loss of engine power near the Natchitoches Regional Airport (IER), near Natchitoches, Louisiana. The commercial pilot and three passengers were not injured. The airplane was registered to a private individual and operated by the pilot. Dusk visual meteorological conditions prevailed, and a instrument flight rules (IFR) flight plan was filed for the 14 Code of Federal Regulations Part 91 personal flight. The cross-country flight originated from the Diamondhead Airport, near Diamondhead, Mississippi, at an unknown time, and was destined for the Center Municipal Airport, near Center, Texas. In a written statement, the pilot reported that while in cruise flight, while assigned at 12,000 feet mean sea level (msl) while on an IFR cross-country flight, he began to smell smoke and noticed the engine began to surge. After requesting a lower altitude, the air traffic controller advised him of the nearest airport. During the descent, at an altitude of approximately 9,000 feet msl, "the engine lost oil pressure" and the pilot shut the engine down using the mixture control. The "engine continued to turn over" as he heard "several loud explosions and observed fireballs at the propeller hub" before the propeller stopped turning. After realizing he was unable to make it to the airport, the pilot elected to land in a field approximately 1/2 mile south of IER. After maneuvering the airplane underneath a set of power lines, the pilot landed in an open field. During the landing roll, the nose gear collapsed and the airplane came to rest upright in a nose low position in a muddy field. A visual examination of the engine by the pilot revealed that there was a hole in the crankcase near the number two cylinder. The 1993-model Bechcraft A36, a low wing six-place airplane, serial number E-2776, was powered by a single Continental IO-550-B engine, rated at 300 horsepower. On December 7, 2005, an examination of the engine was conducted under the supervision of an NTSB investigator at Teledyne Continental Motors, Inc., near Mobile, Alabama. The examination revealed that the engine contained aftermarket parts and components indicating that the engine had been field overhauled. A turbo-normalizing system had also been installed. The crankshaft and counterweight assembly exhibited lubrication distress of varying degrees to all connecting rod journals and bearings. The most severe thermal and mechanical damage was concentrated at the number three connecting rod and journal. The number six piston head exhibited detonation/preignition damage causing the erosion of the piston head edge and approximately twenty-five percent of the radius at exhaust side, torching through and beneath the top ring steel insert and progressing through the remaining rings and lands to the interior. This piston damage would allow excessive by-pass of combustion gases, creating extremely high crankcase pressure, with subsequent oil loss through the crankcase breather system. The installation of a turbo-normalizing system on an engine originally certificated as a naturally aspirated model containing high compression pistons is contrary to the recommendations in TCM Service Bulletin M64-18 and can reduce the engine’s detonation margins. The engine and airframe logbooks were not available for review during the course of the accident investigation.