On August 19, 1999, at 1905 hours Pacific daylight time, a Cessna T210L, N5378V, sustained substantial damage during a forced landing near Byron, California. The forced landing was precipitated by a total loss of engine power during cruise. The aircraft was owned and operated by Auburn Flying Service of Auburn, California, as an on-demand nonscheduled passenger flight under the provisions of 14 CFR Part 135 of the Federal Aviation Regulations. The commercial pilot and five passengers were not injured. The flight originated at the Pine Mountain Lake airport, Groveland, California, and was en route to Concord, California. In a telephone interview on the night of the accident, the pilot said the aircraft was in level cruise at 6,500 feet msl when the engine suddenly quit without warning. The pilot switched fuel tanks and turned on the electric boost pump, which restored fuel flow on the gage, but the engine did not restart. Coincident with the loss of power, the pilot heard a "pop" sound come from the engine compartment. The pilot selected an open field and continued to attempt a restart as he prepared for the off-airport landing. The aircraft touched down in the field and rolled some distance before the nose gear collapsed. During retrieval of the aircraft, an initial assessment of the engine was made. The crankshaft was rotated but no corresponding movement was observed in the vacuum pump drive or magnetos. Very weak compression with crankcase blow-by was noted for the No. 6 cylinder. The mechanic who performed the assessment noted that there was an odd feeling in the crankshaft as it was rotated though the No. 6 cylinder compression stroke. The Continental TSIO-520-HCR engine, serial number 506459, was disassembled for internal examination on August 30, 1999, under the supervision of an Federal Aviation Administration (FAA) Airworthiness Inspector from the Oakland, California, Flight Standards District Office. A complete report of the teardown inspection is appended to this report. External examination disclosed that the crankshaft could only be rotated through about 45 degrees, with no corresponding movement in the accessory gear section. The lower forward cylinder hold down stud for the No. 3 cylinder was broken. The outer end, with nut still attached, was found on the lower cylinder baffle, and the fracture surface displayed semicircular beach markings evident to the naked eye. All cylinder bases and through bolt studs exhibited evidence of oil leakage. During removal of the cylinders, all through bolt and cylinder hold down stud nuts were loose enough to break torque with one hand using a short open-end wrench. A large amount of metal particles were found in the oil sump, which contained 10 quarts of oil. Removal of the cylinders revealed fretted cylinder bases and pads. Orange silicone sealant was found on the cylinder bases and the stud threads. Separation of the case halves revealed fretting on all cylinder pads and bearing saddle-parting surfaces, with more fretting observed on the No. 2 main bearing saddle surfaces. The No. 2 main bearing was fragmented and found in the oil sump, with erosion and smearing evident to the bearing saddle. The crankshaft was fractured across the No. 3 short cheek, with heavy scoring evident to the Nos. 2 and 3 main journals. Review of the maintenance records disclosed that cylinders 1, 2, 4, 5, and 6 were replaced on March 24, 1997, 232 hours prior to the accident.