After landing, a 180 degree turn to the right to backtrack along the runway was planned. As the aircraft was being slowed to taxi speed a gentle left turn to position the aircraft near the left side of the runway was commenced, but the right main gear collapsed. A gear down indication remained on after the aircraft came to a halt. The right main gear retraction push-pull tube bellcrank had developed a fatigue crack in the area of the bushing hole. The bellrcrank failed in overload at this crack causing insufficient downlock tension to be maintained on the extended main landing gear. The aircraft had flown 9336 hours at the time of the failure and the left bellcrank had previously been strengthened.