if there is thread on the head then ur plug just popped out from the vibration you wanna here about exploding engines i blow once a hpi .15 FE engine i ported it like you wouldnt imagine i had that engine HALLIN!!!!!!!lol and i was running it around in the park and all of a sudden i hear BBBBBBOOOOOOOOOOOOMMMMMMMMMMMMM................... i tryed to start it back up but the engine wouldnt turn over so i jsut say screw it and go home and i start taking everything apart..............................the coneting rod snapped and not only did it snap from the looks of it it snapped at TDC and when the piston came back down..........the rod whent threw the piston and the other end of the rod was sticking out of the bottom of the engine case IT BLEW THRU THE CASE!!!!!!!!!!! THATS CRAZY!!!!!!!! that my friends is a exploded engine.