On July 1, 1998, at 1520 eastern daylight time, a Starduster Too, N159MK, operated by a private pilot experienced a right main gear collapse while landing at the Bay City Clements Municipal Airport, Bay City, Michigan. The pilot and passenger were not injured. The airplane was substantially damaged. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions. The flight originated from the Bay City Clements Municipal Airport at 1415 eastern daylight time. The pilot reported that upon landing he lost directional control of the airplane. He stated the airplane swerved on the runway, then skidded sideways off the side of the runway. The airplane then traveled onto a taxiway where it contacted a 3 to 4 inch rise between the grass and the pavement. The right main landing gear collapsed then the rise was contacted. The pilot/builder reported the loss of directional control was a result of an under inflated tailwheel. He reported it should have been inflated to 38 psi due to the extra weight of having a passenger on board. Inspection of the airplane by an Inspector from the Federal Aviation Administration Grand Rapids, Michigan, Flight Standards District Office revealed the right main landing gear separated at the attachment weld. When the right main gear collapsed the lower right wing contacted the terrain. This resulted in a failure of the right lower wing rear spar.