Freeze plugs are a subset of the plugs on a car engine cylinder block or cylinder head. The traditional plug is a thin, domed, disc of metal which fits into a machined hole in the casting and is secured by striking or pressing the centre to expand the disc.
A true Freeze Plug is an expansion plug located in the side of an engine block that is supposed to protect the block against freeze damage. Water expands when it turns to ice, and if the coolant doesn't have enough antifreeze protection it can freeze and crack the engine block. The freeze plugs (there are usually several) are supposed to pop out under such conditions to relieve the pressure on the block.
Most of the plugs to be found on an engine are actually to plug the holes where sand cores have been held. The sand cores are used in the casting process to form internal cavities in the engine block or cylinder head, for cooling water for example, and so should really be refered to as core plugs.
Freeze/core plugs can often be a source of troublesome leaks as a result of internal cooling system corrosion. Ease of replacement depends on accessibility. In many cases the plug area will be difficult to reach and using a mallet to perform maintenance or replacement will be nearly impossible without special facilities. Expanding rubber plugs are available as replacements when access is a problem.
A variety of block heater called a "freeze plug heater" can be installed, replacing the freeze plugs, to warm the engine before start up.
