[olddog]

If all it did to the plug was push the gap closed a bit, I doubt that hurt the piston. The shorter plug should fix this issue.

What would concern me more is the valve to piston clearance. Did you take the time to put clay on the piston, mount head and rotate, then check the clearances, buy the thickness of the clay? If not, I would be really scared right now.

Had you done this with the plugs installed you would have found the problem then. I have to admit, I have no clue if the plugs were installed, when I did this. Note to self, always have the plugs installed that you plan to use, when putting clay on the piston.