On February 21, 1996, about 1400 hours Pacific standard time, a Schweitzer G-164B, N6674Q, nosed over during a forced landing after takeoff from a private ag strip near Tracy, California. The forced landing was precipitated by a loss of engine power during the takeoff initial climb. The aircraft was owned and operated by Haley Flying Service, Inc., of Tracy, and was beginning a local area agricultural application mission. Visual meteorological conditions prevailed at the time and no flight plan was filed for the operation, which was conducted under 14 CFR Part 137. The aircraft sustained substantial damage. The certificated commercial pilot, the sole occupant, was not injured. The flight was originating at the time of the accident. The pilot reported that he was about 1/4 mile from the airport at 50 feet in the initial climb when the engine began to vibrate severely and then abruptly stopped. The pilot selected the only available landing area, which was a muddy field, and the aircraft nosed over during the landing ground roll. Preliminary examination of the engine revealed that the crankshaft was seized and would not move. The engine was then removed from the aircraft and shipped to an overhaul facility, Aero Engines, Los Angeles, California, where it was disassembled under the supervision of an FAA inspector from the Los Angles Flight Standards District Office. The report of the examination disclosed that the engine sustained a catastrophic internal failure and even partial disassembly was extremely difficult. The No. 5 piston (master) was found out of it's respective cylinder, with the connecting rod elongated and wrapped around the crankshaft. The rod end cap was attached on one side and separated on the other. Evidence of heat and stretching was observed near the crankpin area of the rod. The bearing was not observed.