On April 2, 2011, about 1030 central daylight time, a Reeves Avid Mark IV, N6313T, veered off a runway during landing roll and impacted in a drainage ditch at North Central Missouri Regional Airport (MO8), Brookfield, Missouri. The private pilot and passenger were uninjured. The airplane sustained substantial damage to the fuselage. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operating on a flight plan. The flight originated from the pilot's private airstrip near Brookfield, Missouri, about 1010 and was en route to MO8. The pilot said that he changed the tailwheel to a Matco tailwheel, part number WHLT-8W, based upon research he had done on the internet. The change resulted in an increase in tire width. He said that the change to a Matco tailwheel was made so that the tailwheel would "float" during landing rollout on turf runways. He reported that the Matco tailwheel accumulated a total time of 44 hours and 97 cycles. The pilot stated that shortly after a three-point touchdown on runway 36 the tailwheel experienced a severe shimmy, and the airplane started to pull to the left. The airplane continued to move towards the left side of the runway even with right rudder application. The airplane departed the left side of the runway and impacted in the drainage ditch.