One of the most misunderstood parts of an engine block is its freeze plugs. Most people associate the term “freeze” with holes that were designed to offer protection against freezing. If these people knew the other terms freeze plugs are often called, such as expansion plugs and more appropriately core plugs, they might think twice.

What are Freeze Plugs For?

Installed Brass Freeze Plug

Contrary to belief, freeze plugs are not designed to protect the engine block. The freeze plug holes are actually part of the casting so that the original manufacturer of the block can remove casting sand/debris after the block has been molded. Without these openings, it would be extraordinarily difficult to remove sand/debris after the engine block has been cast.

In the picture to the left, you can see a brass freeze plug installed in a small block Chevy engine. Clicking on the picture will allow you to see a larger view, which also shows silicone sealant being used to provide for a positive seal.

Freeze Plug Facts, Repairs and Alternative Uses

If you look inside an engine block, you will see coolant cavities that surround most of the cylinder bores. As the engine operates, the coolant keeps the cylinders and entire block cool. In fact, the coolant cavities around the cylinder walls also feed coolant to the cylinder head passages through the deck of the block. To make sure the coolant cavities and passageways work as intended, freeze plugs must be installed and sealed correctly.

Freeze plugs are made out of a variety of materials that include brass, steel and stainless steel. Each freeze plug is designed to fit a specific hole diameter with a press fit, with some plugs having a convex disc or more commonly seen as a concave cup. Often a high temperature silicone sealer is applied to the freeze plug housing bore so that the newly installed plug will conform to any irregularities and maintain a positive seal. Freeze plugs are often installed with a hammer and a drift. The drift fits inside the freeze plug cup and is gently driven in by hammer until the freeze plug is flush with the housing bore.

If a leaking freeze plug is present on an installed engine, it usually is the result of corrosion that has compromised the plug. This is why brass and stainless steel freeze plugs are typically used on rebuilt engines as they do not rust. In many cases a leaking freeze plug can be repaired by removing and replacing it with an appropriate plug. If access to the leaking plug is limited, where driving in a new freeze plug with a hammer and drift is not possible, an expansion plug is often used for such repairs. Dorman makes many different styles of expansion plugs in various diameters, which can be viewed here. Dorman’s expansion plugs, which are known to be of a high quality in the automotive industry, are rubber and conform to irregular surfaces without the need of silicone sealants. These expansion plugs have a nut on the outside that is tightened so that it can be expanded with a wrench in tight quarters.

Freeze plugs are often used for alternative applications. For example, some block heaters are provisioned to be mounted in an open freeze plug housing bore. Particularly for those in cold climates, the ease of adding an engine block heater on a gasoline or diesel engine is simplified by making use of the block’s original design.

As you can tell by the information above, the true purpose of freeze plugs is often misunderstood. Regardless, these plugs are an important part of an engine block. If you would like to learn more about engine blocks and their related components, please click here for even more detailed information.


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Freeze Plug Facts