When advising replacing a thermostat. Shouldnt you guys advise how to tighten the thermostat housing down so it does crack?
If the thermostat housing was made of plastic (carbon fiber) and the temperature outside dropped below the freezing point of the cooling system, then the coolant would have froze up and expanded causing the thermostat housing to crack.
Place a new thermostat gasket on the engine block. Place the thermostat housing over the block and align the screw holes. Apply some anti-seize lubricant to the mounting bolt threads. Screw the mounting bolts in by hand, then tighten them firmly with a socket, extension and wrench.
It is not 'unsafe' in terms of road safety. The cooling system may well use coolant as you drive and if it runs out of coolant the engine will overheat and possibly seize up (then you'll need a full engine rebuild).
If it is coming from the thermostat location in the housing, it could simply be a thermostat gasket/O-ring leaking. A cooling system pressure test would uncover the leak location. Decreased coolant in the cooling system can result in overheating and possible engine damage.
But after a bit of research, I discovered that replacing a thermostat is pretty stinking easy. Granted, each heating and air conditioning system will have unique requirements for connecting a new thermostat, but the general process is pretty much the same. Consider this your big picture guide through the process.