[Jimmyb]

Core plugs, are usually metal cups that fill the sand casting core holes found on water-cooled internal combustion engines. They are often wrongly called Welsh plugs, frost plugs or freeze plugs.[1]
Sand cores are used to form the internal cavities when the engine block or cylinder head(s) is cast. These cavities are usually the coolant passages. Holes are left in the casting to facilitate the removal of the sand after the casting has cooled.
In some high-performance engines the core plugs are large diameter pipe plugs.[2]
Core plugs can often be a source of leaks due to corrosion caused by cooling system water.[3] 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.