On August 20, 1995, at 1415 central daylight time (cdt), a Cessna 172P, N55404, operated by Waukesha Flying Service, Inc., of Waukesha, Wisconsin, was substantially damaged following a loss of engine power and subsequent forced landing in Port Washington, Wisconsin. The commercially certificated pilot and three passengers reported minor injuries. The local 14 CFR Part 91 pleasure flight departed Waukesha County Airport, Waukesha, Wisconsin, at 1330 cdt. No flight plan was filed and visual meteorological conditions existed at the time of the accident. In a detailed written statement, the pilot reported the engine began to run rough approximately 45 minutes after departure. The pilot stated that he immediately applied carburetor heat and performed a magneto check, but the engine roughness became progressively worse. The pilot stated that the oil temperature gauge showed a rapid increase, the oil pressure gauge showed a rapid decrease, and white smoke began to emit from the front of the engine cowling. The pilot selected a field to make a forced landing on and briefed the passengers. After clearing tension wires which lined the approach end of the field, the airplane touched down on the field. The nose wheel dug into the soil and the airplane came to rest in an inverted position. Postaccident investigation revealed the number two cylinder, P/N 78475, had fractured. The cylinder was sent to the NTSB Materials Laboratory, Washington, D.C., for examination. The laboratory reported the cylinder barrel was cracked through an arc of approximately 300 degrees. The largest opening of the crack was between the fourth and fifth cooling fins. Examination of the cleaned fracture revealed crack arrest positions of fatigue cracking. See attached NTSB Metallurgist's Factual Report and Textron Lycoming Technical Report. The airplane logbook shows the last inspection was an annual inspection performed on August 8, 1995. The airplane has logged 19 hours of flight time since the inspection.