The pilot reported that he had obtained normal indications when he selected the landing gear down. The subsequent touchdown was normal but as the nosewheel was lowered to the ground, the nose gear leg collapsed. The eyebolt between the nose gear retract aft rod and the landing gear motor had failed, resulting in the nose gear failing to lock down and allowing it to retract on ground contact. The eyebolt had failed at the beginning of the thread, adjacent to the eye. Specialist examination indicated that the initial fracture was indicative of fatigue and had been present for some time. The aircraft had only recently been returned to service following an earlier accident which involved a fatigue failure of the nose gear retract rod plunger fork. It was considered likely that the fracture had initiated at the same time as the earlier failure.