Radiator caps and pressure

[Rubbond]

Kevin,
It's precisely to stop the coolant boiling! Otherwise you would have to keep
topping up.
Many engines are designed to run at about 105 degC, adding to the pressure
requirement.
Coolants contain many additives, but the most important are glycols (I
believe mostly ethylene di-glycol), to lower the freezing point, and
additives to supress corrosion. Not sure what their boiling point/pressure
diagrams look like, but no doubt the radiator cap resistance is designed to
be higher than the type of coolant recommended. In Canada people will use
higher concentrations of glycols, but leave the same coolant in the engine
in the summer.
Still, I can't see why these plugs should blow out!
Andrew.