On July 27, 1998, about 0800 eastern daylight time, a Cessna A-188B, N21676, registered to a private individual, operating as a 14 CFR 137 aerial application flight, crashed near the town of Zolfo Springs, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The airplane received substantial damage and the commercial-rated pilot, the sole occupant, was not injured. The flight originated about 25 minutes before the accident. The pilot stated he had just pulled off his second tract of the day, at about 100 to 150 feet of altitude agl, when he heard a "clunk" and his engine quit. At such an altitude, he stated his only option was a forced landing to a "wide row" in the citrus grove. Subsequent disassembly inspection of the engine, with FAA overview, revealed the initial failure to be the migration of one of the two counterweight mounting pins on the rearmost crankshaft journal. This resulted in flailing of the counterweight until it broke through the crankcase. The #5 and #6 cylinder barrels had been peened inside the crankcase and had to be sawed off. The crankshaft had broken. The accident happened at the aircraft's tachometer reading of 2929.2 hours. The engine underwent a major overhaul and was signed off by an authorized inspector on August 27, 1997, at a tachometer reading of 2678.8 hours, or about 250 hours previous to the accident. The aircraft had undergone a 100 hour inspection on February 17, 1998, at a tachometer reading of 2744.5 hours. Copies of engine and aircraft log entries are included under, "Other Pertinent Forms and Reports".