On February 20, 1999, at 1045 central standard time, a Cessna T210R, N7687U, piloted by a commercial pilot, sustained minor damage during a forced landing near Tinley Park, Illinois. The propeller separated from the airplane in-flight. The pilot and two passengers reported no injuries. The business flight was conducted under 14 CFR Part 91. Visual meteorological conditions prevailed. No flight plan was on file. The flight departed Capitol Airport, Springfield, Illinois, at 0945 and was en route to Meigs Field, Chicago, Illinois. In a written statement, the pilot said that in-flight at 1,800 feet above mean sea level, the engine seized. He stated that the propeller bolts sheared, the propeller fell to the ground, and that he landed the airplane without any damage to the airplane. An annual inspection had been performed on March 9, 1998. The airplane accumulated approximately 288 tachometer hours since that inspection and approximately 85 tachometer hours since a top overhaul was performed. On March 2, 1999, a preliminary borescope inspection performed at Lumanair, Aurora, Illinois, revealed that the number three and four connecting rods were broken and that they were colored blue. A crankshaft counter balance was found lodged against the side of the crankcase forward of the number four cylinder. One quart of oil was found in the sump. The number four cylinder head was found separated from its barrel. Three of the studs that held the propeller to the crankshaft flange were observed to have their fracture initiate on opposite sides of those studs. The crankshaft flange and propeller hub mating surfaces were found with areas having a polished appearance. On May 10, 1999, the engine was inspected at Teledyne Continental Motors, Mobile, Alabama. A dark, oily substance was observed coating the exterior surface of the number one and three cylinders. The turbocharger was examined and its pressure oil line "B" nut was found loose. A dark, charred, oily residue was found on the turbocharger and its housing. The engine was disassembled. The oil sump was found to contain metallic material. The oil pump gears exhibited roughness and corrosion. Metal debris was found in the oil filter. All cylinder overhead valve components were found intact except for the number four cylinder which had its cylinder head separated. The number one, four, five, and six crankshaft rod journals revealed discoloration. The number one crankshaft journal was found blue. The number four, five, and six rod journals had a material deposited onto their journal. The deposited material was consistent in color and appearance to bearing material. The inside diameter of the oil transfer collar was found with corrosion. Cylinder crowns and piston crowns were found with a brown to black coloration. The number four cylinder head was found separated from its base. The surface of the separation was rough and irregular.