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Expansion plugs, also referred to as core plugs, welch plugs, and freeze plugs, are thin sheets of metal formed in a cup-like shape. They are commonly made from mild steel with zinc coating for corrosion resistance. These core plugs are used to seal holes in water-cooled internal combustion engines and may pop or expand in increased internal pressure due to overheating or expanding frozen coolant. Expansion plugs are used in most diesel and petrol combustion engines.