The pilot reported that the accident occurred at the end of the sixth leg for the day. The landing gear had been working normally. He had obtained a safe landing gear indication prior to landing. The right landing gear collapsed as the aircraft slowed after the touchdown. Inspection indicated that the torque link had failed in overload. Subsequent engineering investigation indicated that a torque link retainer circlip had fallen off. The torque link's action had prevented the oleo piston from falling out until the link itself failed. A similar problem had been detected during a scheduled inspection completed 20 flying hours prior to the accident. The clip was found to have fallen off and become stuck in the landing gear upper trunnion. The gear was only dismantled sufficiently for the clip to be replaced. The factors that led to the circlip falling out were not determined at the time of the scheduled inspection, and could not be determined after the accident as the circlip was not recovered.