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A core plug, or freeze plug as it is better known, is a device used to allow water room to expand as it freezes, thereby avoiding breaking the engine block or cylinder heads. Made of steel or brass, the core plug is driven into position with a blunt driving tool and held in place with a thin layer of adhesive or sealant. When the engine block and cylinder heads are being made from either cast iron or aluminum, several large openings are cast into the water jacket of the pieces. Installing a core plug into each of these openings creates an easy-to-dislodge path for any expanding ice to take, which will leave the engine intact and unbroken.
Often referred to as expansion plugs, the title refers to the ability of the core plug to be pushed out of place, allowing any damage from expanding ice to be avoided. Occasionally, the thin metal core plug will rust through, resulting in a coolant leak. Depending on the location of the leaky core plug, the engine may have to be removed from the vehicle to make repairs. Certain manufacturers place a core plug on the rear of the engine block, requiring the transmission to be separated from the engine block in order to replace the plug.
In high-performance engines that will seldom see highway use, expansion plugs are often replaced by threaded plugs. The threaded pipe plugs offer added strength to the engine block over the thin glue in plugs. Many times, these high-horsepower engines have the water jacket filled with a cement-like substance known as block fill. This filling of the engine block's water jacket offers added strength to the cylinder sleeves by placing solid material behind them and not simply coolant. The downside of this practice is that the fill can never be removed and, thus, the engine will never be able to have coolant run through it again.
In emergency situations, there is an expandable core plug that can be placed in a leaking plug's place. This expandable plug can be placed between two steel washers with a bolt running through the entire assembly. The old leaking plug is removed by forcing it out of the engine block with a screwdriver. The temporary core plug is placed into the vacated plug opening, and the bolt is turned with a wrench, causing the rubber to be pushed outward and locking the plug into position. The temporary plug should be replaced with a permanent plug at the earliest convenience.