I’ve gone through 12 spark plugs (cyl 1-4) and one helicoil on cylinder 3. Cylinder 3 is on its third spark plug since December 2017 and the plug in the picture on the right I just took out of cylinder 1 tonight. Same thing happened to the plug in cylinder 3 a few months ago, and I never found the plug and the boot that blew out of cyl 3 a few weeks ago. What causes spark plugs to break like this (plug on right)? Misadjusted valves?