The airplane was in cruise flight at 6,500 feet when the engine began to run roughly. The pilot verified both magnetos were functioning properly, switched fuel tanks, and manipulated the throttle without any improvement in engine operation. He informed air traffic control of the loss of engine power and was subsequently cleared to land at an airport about 8 nm north of his position. The engine continued to run roughly as he maneuvered for the airport. The airplane impacted rising terrain that preceded the runway threshold. The airplane came to rest within 100 feet of the threshold. The airplane received substantial damage to the right wing and engine firewall. The nose and right main landing gear collapsed during the landing. The left main landing gear remained fully extended. Examination of the engine revealed that the number 3 cylinder had separated from the engine case. Further examination showed that the number 3 connecting rod had separated from the crankshaft. The connecting rod cap, two rod bolts, and two nuts were found separated inside the engine case. One of the rod bolts was bent and fractured approximately mid-span. One-half of this bolt remained with the rod cap. The bolt's fracture surface exhibited a cup/cone shape, consistent with an overload failure. The threaded region of this bolt was undamaged. One of the nuts had undamaged threads. The first three threads adjacent to the flat side of the remaining nut were damaged. The remaining rod bolt was intact, but the three threads from the end were damaged consistent with the damage noted on the second nut. The last engine overhaul was completed on November 19, 1972. The engine had accumulated 820 hours since that overhaul.