Core plug

Core plugs are used to fill the sand casting core holes found on water-cooled internal combustion engines. They are also commonly called frost plugs, freeze plugs, or engine block expansion plugs. Core plugs are used to fill the sand casting core holes found on water-cooled internal combustion engines. They are also commonly called frost plugs, freeze plugs, or engine block expansion plugs. 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 designed into the casting to support internal sand forms, and to facilitate the removal of the sand after the casting has cooled. Core plugs are usually thin metal cups press fitted into the casting holes, but may be made of rubber or other materials. In some high-performance engines the core plugs are large diameter cast metal threaded pipe plugs. Core plugs can often be a source of leaks due to corrosion caused by cooling system water. Although modern antifreeze chemicals do not evaporate and may be considered 'permanent', anti-corrosion additives gradually deplete and must be replenished. Failure to do this periodic maintenance accelerates corrosion of engine parts, and the thin metal core plugs are often the first components to start leaking. Difficulty or ease of core plug replacement depends upon physical accessibility in a crowded engine compartment. 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 for partial or complete removal of the engine. Specialized copper or rubber replacement plugs are available which can be expanded by using a wrench when access is a problem, though engine removal may still be required in some cases.