it sounds to me like there is something in that cylinder.
If the rod is bent,then the piston will not come up as far,so it would'nt be hitting the plug.
I would not run it any more and would bring it to a shop to have them look in the cylinder with a borescope to see if anything is in there.If you keep running it,it may cause serious damage.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."