- Freezeout plug
Freeze plugs are mounted on the outer wall of the engine. These are alloy plates that seal holes in the engine's internal cooling channels. When casting the engine block, internal molds are held in place via these holes and after production the dust and grindings are removed from the cooling channels. An additional advantage is that when the coolant freezes (when the antifreeze level is too low), these plates can freeze to pieces. The cooling channels can thus be protected against cracks, because the pressure of the frozen cooling water finds its way out. This advantage has given it the name "freeze stop". However, it can sometimes go wrong that the block still cracks due to freezing. The antifreeze content of the coolant is then much too low, or it is due to an extremely cold climate. See here more information about antifreeze, or Ethylene glycol.
The disadvantage of a freeze stop is that they sometimes start to leak. Coolant then runs out or mixes with the engine oil. They are usually easy to replace separately.