not enough thread depth, overtorque, fatigue of the aluminum. Could be any of those reasons. I've repaired heads that have blown out original plugs with 60k on the clock. but Ive also seen replacement plugs blow out... and the plug gets beat up from banging around in the s.p. well just like Blueline said. The 2v motors have a small bolt holding the coil down and there's enough force to break the plastic tab off the side of the coil. And it splits the boot open like a hot dog in the microwave.