On March 18, 2010, approximately 1455 Pacific daylight time, a Carr RV-4 experimental amateur-built airplane, N66PB, sustained substantial damage when the left main landing gear collapsed during landing rollout at the Arlington Municipal Airport, Arlington, Washington. The airline transport pilot, the sole occupant, was not injured. The owner/pilot was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the personal flight, which had originated from Burlington, Washington, approximately 1430. A flight plan had not been filed. The pilot said that when he took off and retracted the airplane's landing gear, only the tail wheel retracted. He lowered the landing gear, and got three green lights indicating that all three landing gear were down and safe for landing. He then proceeded to Arlington, where during the landing roll out on runway 34, the left main landing gear collapsed. The airplane departed the left side of the runway and subsequently the right main landing gear collapsed. When the left wing's main landing gear collapsed outward, it damaged the wing structure and punctured the left fuel tank. The skin on the top of the right wing and the right aileron were wrinkled. Postaccident examination of the landing gear revealed that both main landing gear drag link bolts failed. These bolts were welded to the back of each main landing gear as part of their landing retraction system. Examination of photographs of the broken bolts by an NTSB metallurgist revealed that each bolt exhibited cantilever beam bending in overstress. The quality of the photographs was inadequate to determine if fatigue played a role.