Sorry for the late reply, I tried to send a reply message, but it got lost
along the way, so I am trying again.

The plug cracked in the outer metal case. The ceramic part inside looks
fine. The crack is smooth and slightly curved and extends longitudanlay from
almost from one set of threads to the other. I will try to put a picture up
if anyone is still interested. I don't think this could have been caused by
over torquing, because this usually creates a crack at an angle to the
longitudianal axis. I also think it is unlikely to have been caused by
dropping it since there is no evidence of an impact. I am thinking it was
caused by cheap metal and repeated temperature cycles, but then again I am
not a meteurlagist.