The pilot receiving instruction during the instructional flight reported that he performed a wheel landing and the airplane touched down on the runway centerline. After the tailwheel touched down onto the runway, the airplane began to track to the left. The pilot began to apply increasing right rudder control input, but the airplane continued to track left. He then applied right “differential steering” and the airplane overshot the runway centerline, tracking right. He was unable to arrest the right track with full left rudder control input and left brake, and the airplane ground looped. The left landing gear collapsed, and the left wing struck the ground. The airplane sustained substantial damage to the left wing spar. A postaccident examination of the airplane revealed that the tailwheel was out of rigging specifications, which would have resulted in a loss of directional control during landing.