What is a core plug?

What is a core plug?

A core plug is a thin piece of metal either in a cup or dish shape which is used to block off a hole or some form of opening on an engine block or cylinder head that was made in the manufacturing process. Most common types of openings can be coolant castings on the engine block or drilled oil galleries within the engine block or cylinder head.

Other common names for core plug are welch plug, freeze plug and expansion plug.

Core plugs are fitted to nearly all diesel and petrol automotive, marine and industrial engines.

Materials vary including mild and stainless steel, brass and aluminium.

Most common are mild steel core plugs which are usually finished with a zinc-nickel electroplating finish to form a corrosion resistance against fluids and high temperatures once fitted to the engine or cylinder head.

It is common practice when rebuilding or overhauling an engine block or cylinder head to remove all core plugs to clean the all the coolant and oil gallery areas thoroughly especially if the engine block or cylinder head have been soda or shot blasted. All the core plugs are replaced when it is being built back up.