According to the pilot, he landed the amateur-built airplane on runway 18 and during the rollout, it slowly veered to the left. The airplane ground looped and then departed the left side of the runway. The right main landing gear became caught in a grassy area, which brought the airplane to rest and resulted in substantial damage to the fuselage. An examination of the airplane by a Federal Aviation Administration inspector revealed the tailwheel lock mechanism of the free-castering tailwheel assembly was unable to keep the tailwheel engaged in the locked position. The mechanism that locked the tailwheel assembly in place was not broken; however, the spring of that particular tailwheel assembly kit worked in compression, rather than tension, which resulted in less pressure applied to the tailwheel lock and greater susceptibility for the tailwheel to unlock during a crosswind landing. Subsequently, when the inspector moved the lock mechanism repetitively, it became dislodged from the notch. Wind at the airport about the time of the accident was from 110 at 5 knots.