Freeze Plug
A metal cup pressed into engine block casting holes designed to pop out or corrode before the block cracks from frozen coolant.
A freeze plug (more accurately called a core plug or casting plug) is a small metal cup pressed into holes in the side of the engine block or cylinder head. These holes are originally created during the casting process to allow sand to be cleaned out of the internal coolant passages.
While they are named freeze plugs because they are intended to pop out if the engine coolant freezes (preventing the expanding ice from cracking the cast-iron or aluminum block), their primary modern role is simply to seal these casting holes. Over time, if the engine coolant is not changed regularly, corrosion can eat through the thin metal plugs, causing them to leak.
A leaking freeze plug will cause a persistent coolant leak, often in hard-to-reach areas behind the exhaust manifold, engine mounts, or transmission. Replacing a freeze plug requires cleaning the casting hole, applying sealant, and driving a new brass or steel plug in using a specialized tool.