( stud bolt), going into the block and tightening up the whole thermostat unit has gone from the block end, you cannot put the bolt in the bottom hole as the thread that it goes in has snapped.. if that makes sense... 
I have heard you can cast a new mould for the stud bolt to go into using quick steel
( putty that turns into metal after you play with it in your hands).This is risky as it could still leak if you dunno get the bolt flush so no water/coolent comes out
..Any advice??