On April 8, 1997, about 1630 eastern daylight time, a Cumberland Skybolt, N4TZ, registered to an individual, nosed over while making a forced landing following loss of engine power at Smithfield, North Carolina, while on a Title 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the private-rated pilot and one passenger were not injured. The flight originated from Walterboro, South Carolina, the same day, about 1400. The pilot stated he was inbound to Johnson County Airport at 1,100 feet. About 1.5 miles from the airport the engine failed and stopped turning. He elected to land in a field. While making a forced landing in a freshly plowed field the aircraft nosed over. Postcrash examination of the engine was performed by an FAA inspector and a representative from Lycoming Engines. The top of the engine case was broken out from the No. 4 cylinder rearward. Disassembly of the engine showed that one counterweight roller, washer, and retaining clip were separated from the counterweight and the counterweight was free to move around on the remaining roller. The separated components were found within the engine. The No. 5 connecting rod adjacent to the loose counterweight had been impacted by the counterweight and failed about 3 inches from the crankshaft end. Metallurgical examination of the separated counterweight washer and retaining clip was performed the Materials Laboratory, Lycoming Engines. The counterweight washer had two pieces broken from it, which were not recovered. The fracture surfaces were damaged and the failure mode could not be determined. The washer did not meet engineering drawing requirements. Examination showed the washer had been ground on one side, a procedure not allowed by Lycoming process specifications. The counterweight clip had one end piece missing from it, which was not recovered. The clip conformed to engineering requirements. (See Lycoming Engine Report) The pilot stated he and his family took the engine to the Frederick Community College, Aviation Maintenance Program, in March 1994, for overhaul by the students. They received the completed engine in May 1995, and installed it on N4TZ. The engine had accumulated 83 flight hours from the time of installation to the time of failure. They did not receive logbook records from Frederick Community College showing the engine had been overhauled. (See Statement) The student who performed the work on the engine for N4TZ stated he was asked if he "would be interested in taking on the responsibility for rebuilding a Lycoming O-540 aircraft engine as an after hours special project." He agreed to undertake the re-building of the engine, which would be done under direct supervision of Mr. Neil Wright, the reciprocating engine instructor at the school, with oversight provided by Mr. Gerard Blake, the head of the school. He stated that after all the work was completed the contents of a loose leaf repair record was explained to the owner and his son, and the book, as well as all remaining parts and supplies were turned over to them. He does not know what happened to the original logbook for the engine or who signed off the logbook as to the status of the engine. (See attached statement) Mr. Gerard Blake of Frederick Community College stated to FAA after the accident that work performed on the engine was only a repair and not an overhaul, and that he did not know what happened to the engine logbooks. (See FAA Inspector Statement)