On January 24, 1999, about 0900 hours Pacific standard time, a Hiller UH-12E, N67064, operated by Breen Aviation, Inc., experienced a total loss of engine power while maneuvering during an aerial application flight about 7 miles north of Lemoore, California. The flight was being conducted under the provisions of 14 CFR Part 137. The pilot reported that he heard a noise emanating from the engine. Thereafter the engine seized, and the pilot stated he made a forced landing from his altitude of 15 feet above ground level. Following touchdown in the soft, uneven alfalfa field, the helicopter rolled over and was substantially damaged. The commercial pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from a nearby field about 0800. The helicopter was recovered and examined by Federal Aviation Administration (FAA) airworthiness inspectors from the Fresno, California, Flight Standards District Office. Maintenance records indicated that the helicopter's engine had been operated about 482 hours since being overhauled. The FAA inspectors reported that the engine's number two connecting rod was found broken at its crankshaft end, and the break did not appear related to the rod bolts. Additionally, an inspection of the associated cylinder and piston showed signatures consistent with detonation. Similar signatures indicative of exposure to elevated temperatures were observed with the other five piston heads. Maintenance discrepancies were also noted with the primer line and magnetos. A small hole was found chaffed in the primer line. The magneto distributor blocks for both magnetos showed evidence of carbon and burning of the rotor fingers. The magnetos were subsequently bench tested and were found functional.