On May 30, 2007, about 1550 central daylight time, a Cessna 340A, N17MH, piloted by a private pilot and flight instructor, was substantially damaged when the right main landing gear collapsed during landing on runway 28 (4,000 feet by 150 feet, asphalt) at the Middleton Municipal Airport (C29), Middleton, Wisconsin. The local proficiency flight was operating under 14 CFR Part 91 without a flight plan. Visual meteorological conditions prevailed at the time of the accident. The pilot and flight instructor were not injured. The flight originally departed C29 and executed multiple instrument approaches and landings at local airports prior to the accident. The takeoff prior to the accident was from the Dane County Regional Airport (MSN), located approximately 10 miles east of C29, about 1540. The private pilot reported that the objective of the day's flying was to regain proficiency in Cessna 340 operations under the supervision of a flight instructor. They conducted multiple approaches and landings at local airports prior to the accident. Regarding the accident landing, the private pilot stated that during the approach he observed three green lights indicating that the landing gear was properly extended. He also noted that the landing gear warning horn was silent. The landing was "light, at blue line, on track" with proper inputs for a left crosswind, according to the private pilot. He reported that a "few seconds" after touchdown the aircraft began to "shudder" and the gear horn started to sound. He noted that the right main landing gear light was no longer illuminated and the red gear unsafe light was illuminated. The private pilot stated that the aircraft began to drift right and the right wing began to settle. As the right main gear collapsed, the airplane ultimately departed the runway pavement, striking two runway lights in the process. The airplane came to rest in a grass area adjacent to the runway. The nose and left main landing gear remained extended during the accident. The fuselage was punctured by blade fragments from the right propeller assembly. A post accident examination of the airplane revealed that the forward bellcrank attachment flange on the landing gear strut had failed. In addition, the lower flange on the bellcrank had also failed. No other anomalies were noted. The remaining landing gear assemblies remained intact. Review of the airplane maintenance records revealed that the most recent annual inspection was completed on January 12, 2007. At the time of that inspection, the airplane had accumulated 4,575 hours total flight time. The maintenance log entry indicated that the landing gear system was inspected and adjusted at that time. The right main landing gear downlock tension did not initially meet the specification when checked. The maintenance log entry stated: "Check gear tensions, right main [too] high, adjust torque tube push rod and adjust down lock arm length . . . shorten down lock arm and pops in locked position correctly." The airplane service manual stated that landing gear downlock spring tension was to be inspected at the Side Brace Lock Link/Bellcrank joint. In the event that the tension was too high, the corresponding push-pull tube was to be lengthened until the correct tension was obtained. Any change in the push-pull tube length would normally require a corresponding adjustment to the fork bolt, in order to maintain the proper landing gear position when retracted. According to the airplane manufacturer, the location of the failures was consistent with the cumulative effect of improper adjustment of the downlock mechanism over time. However, any improper adjustment procedure could not be attributed to any specific maintenance action or to a particular mechanic. The maintenance log entry related to the adjustments made during the most recent annual inspection appeared to comply with the airplane maintenance manual instructions.