On January 6, 2017, about 1720 Pacific standard time, a Cessna 421C airplane, N71RC, sustained substantial damage to the right wing spar following the collapse of the right main landing gear during the landing roll at Westover Field/Amador County Airport, Jackson, California. The airline transport rated pilot was not injured. The airplane was registered to and operated by the pilot as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed, and no flight plan was filed for the cross country flight. The flight originated from the Palo Alto Airport, Palo Alto, California about 1640. The pilot stated that he flew a normal left-hand traffic pattern for runway 19. The flare and initial touchdown were normal, however, immediately after the initial touchdown, something did not feel right. When he attempted to lightly apply the brakes the airplane began to yaw from one side to the other. He released the brakes and attempted to slow the airplane with reverse thrust, but the airplane exited the paved runway surface and came to a stop in the grass and dirt area about 20 ft off the right side of the runway. During the landing the right main landing gear collapsed. An onsite examination of the airplane revealed that the right main landing gear torque link assembly had separated at the hinge attach point due to the failure of the torque link bolt. The bolt's nut, cotter pin and washer were not located during the investigation. The subject bolt was subsequently shipped to the National Transportation Safety Board's Materials Laboratory in Washington, D.C. for examination. As a result of the examination, the report revealed that the torque link bolt threads had been rounded over and exhibited remnants of material consistent with the threads of the mating nut. Further, remains of a cotter pin that was installed through the bolt hole were also visible. The report concluded that the deformation of the cotter pin remains was indicative of a shear overstress fracture. (Refer to the NTSB Materials Laboratory Factual Report No. 18-062, which is appended to the docket for this report.) It was revealed during the investigation that Federal Aviation Administration (FAA) Special Airworthiness Information Bulletin (SAIB) CE-14-19, dated May 30, 2014, titled "Landing Gear; Main Landing Gear (MLG) Torque Link," was published to advise owners, operators, and maintenance technicians of Cessna 300 and 400 Models and Series of an airworthiness concerns, specifically the need to inspect the MLG torque link assembly to ensure the correct thickness washers are installed, and to ensure the washers are installed properly. The SAIB was preceded by Cessna Service Bulletin ME83-37, dated November 23, 1983, "Main Landing Gear Scissor Washer Replacement which recommended replacement of the two outer washers on the main landing gear scissor link for Cessna Model 421s, and Cessna Service Bulletin MEB02-12, which specified replacement of the original washers at each end of hinge point attachment stack-up for Cessna Model 310s. (Refer to FAA SAIB CE-14-9, and Cessna Service Bulletin ME83-37, both of which are appended to the docket for this report.) Except for the right main landing gear torque link bolt, no other components of the right main landing gear assembly were accounted for during the investigation. Additionally, no maintenance records were recovered during the investigation for review to determine compliance with the SAIB's. The pilot did not submit a Pilot/Operator Aircraft Accident/Incident Report (NTSB) Form 6120.1.