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Replacing a Freeze Plug

Expanding on the expansion plug

A freeze or expansion plug is a small, metal, circular plug that lives in various places on an engine block. These plugs have a valuable function and an equally interesting origin. An engine block starts life as molten metal. In order to form an engine block, this metal is poured into a mold. When the metal cools off from a liquid to a solid, the engine block is born. As the mold is no longer needed, it is knocked away from the engine block. As most modern engines are liquid cooled, part of this mold also forms the cooling passages inside the engine and must be knocked away as well. The cooling jacket mold material is removed through the holes now filled by the freeze plugs.

Keep Your Cool

Along with filling holes, the freeze plugs have another function. Water expands when frozen. Metal on the other hand does not like to expand very much. If for some reason the liquid coolant inside the engine block freezes and expands, the freeze plug is designed to pop out of the engine block to allow coolant to expand out of the hole. The inexpensive freeze plug can save thousands of dollars in cracked engine blocks. While all this is fine and good, freeze plugs will sometimes leak and fail for reasons that have nothing to do with cold weather. Neglected engine coolant becomes corrosive and can eat away at freeze plugs from the inside out and cause a leak. For these two reasons, maintaining engine coolant is important—on the one hand to prevent corrosion from forming, and on the other to maintain the correct level of anti-freezing properties during sub-freezing cold spells.

Holey Moley

While replacing a freeze plug in itself is fairly simple, getting to it may be another story. In fact, this story can be a long one. As bad luck will likely have it, the leaky freeze plug will never be the one that is easier to see than the sun at noon on a summer day. The leaking freeze plug will be the one up against the back of the firewall or underneath nearly every other part connected to the engine. The additional unfortunate reality is that if one freeze plug has gone rusty with holes then the others are likely not far behind. The best time to replace freeze plugs is when the engine block is out of the car and up on a stand. If this is not an option then digging in and replacing that one leaking freeze plug may be the only answer.