On May 8, 1997, at 0714 central daylight time, a Cessna 210, N7398M, operated as Prompt Air Flight 752, sustained a total loss of engine power during cruise flight near Winona, Minnesota. The commercial rated pilot conducted a forced landing at the Max Conrad Airport, Winona, Minnesota, without further incident. The pilot was not injured. The airplane was not damaged. The 14 CFR part 135, unscheduled air taxi flight departed Milwaukee, Wisconsin, at 0550 with a planned destination of Minneapolis, Minnesota. Visual meteorological conditions prevailed and an IFR flight plan was filed. The pilot reported that while flying at 8,000 feet mean sea level, about a half hour into the flight, the engine missed then ran normal again. He reported the oil pressure was at the bottom of the green arc, the oil temperature was running high, but still in the green arc, and the cylinder head temperature was high, but also still in the green arc. He stated the engine then lost partial power with the manifold pressure dropping to around 20 inches. He stated he informed air traffic control of the situation and requested the nearest airport which was the Winona Max Conrad Airport, 8 miles away. The pilot stated he turned toward the airport and established the best glide speed. He stated he applied full mixture, put on the fuel pump, switched fuel tanks, and tried switching magnetos to no avail. The pilot stated he pulled out the emergency checklist and used the restart and engine securing procedures. Air traffic control then instructed him to contact the unicom frequency at which time he stated he had the airport in sight. He stated he lowered the gear and flaps, and on short final the engine lost all power. The pilot landed the airplane on runway 29 without incident. The airplane was equipped with a Continental TSIO-520-R engine, s/n 517445. The engine was removed from the airplane and shipped to Teledyne Continental Motors, Mobile, Alabama, for a teardown inspection. The teardown inspection revealed that the wrong pistons were installed in the engine. The pistons should have been part number 648044 (compression ratio of 7 to 1). The pistons found installed in the engine were part number 648045 (compression ratio of 8.5 to 1). See teardown inspection reports for further details. The engine was overhauled on July 30, 1996, by G & N Aircraft, Griffith, Indiana. The last inspection on the engine was a 100 hour inspection on May 2, 1997 (6 days prior to the accident) at which time the engine had 736.6 hours since the overhaul.