i recently talked to a guy who had the same problem, he bored out w new piston...... and it still did it so he took it to his mechanic, and i guess the cdi box had failed causing an improper timing and firing. being that it fired at the wrong time, it caused the piston to ''jerk'' wearing out the piston skirt and actually wearing out the bottom of the cylinder, just enough to cause that piston slap. he said when a new cdi box was put in after another new bore the piston slap had stopped. i dont know if this is true or not so any comments would help