Frost plug

A frost plug, also known as a core plug, frost plug, or freeze plug, is a metal (sometimes rubber) cap found in the engine blocks of combustion engines. They are usually located on the side of the engine block or cylinder head.
Freeze plugs were originally designed to prevent the engine block from cracking when the coolant freezes. Frozen water expands, and if there is no room for it to expand, the cast iron engine block can crack. The freeze plug is a weak point that can “pop” out, releasing the pressure.

During the casting of an engine block, core pieces are placed to form the internal cooling channels. After casting, openings remain in the block that need to be sealed—this is where the freeze plugs go. Sometimes freeze plugs can leak due to corrosion or age. In that case, they need to be replaced, otherwise it can lead to coolant loss and engine damage.