Core plug

A core plug, also known as a freeze plug, expansion plug, or Welch plug, is a small metal or rubber disc used to seal core holes in the engine block and cylinder head of an internal combustion engine. These holes are created during the casting process to allow sand and other core materials to be removed after the casting has cooled.

During engine manufacturing, molten metal is poured around sand cores that define the internal passages of the engine block, such as the coolant passages. Once the metal solidifies, the sand cores need to be removed. Core holes provide access points for this process.

After the cores are removed, the core holes must be sealed to prevent coolant or oil from leaking out. Core plugs are pressed or hammered into these holes to create a watertight seal. They are typically made of steel, brass, or rubber.

The term "freeze plug" is a misnomer. While core plugs can be pushed out by expanding ice if the coolant in the engine freezes, they are not specifically designed to act as a safety release valve in such situations. Modern antifreeze solutions are designed to prevent freezing at typical operating temperatures, mitigating the risk of core plugs being forced out by ice formation. Their primary function remains to seal the core holes created during the casting process.

Failure of a core plug can result in significant coolant loss, leading to engine overheating and potential damage. Replacement of a leaking core plug often requires removing surrounding engine components for access, and can be a challenging repair.