Replacing Frost Plugs: Bringing A Car Out of Long-Term Storage

Proving the case that classic cars can break down even when you’re not driving them, frost plugs or also known as expansion plugs or core plugs are known to fail even when a car is in storage. The correct term for these plugs in the engine’s cooling jackets is “Core Plug” although “Frost Plugs” is more commonly used.

The thought was that if an engine’s antifreeze protection was inadequate the plugs would provide a “weak link” to prevent damage to the engine block as the freezing coolant expands. The truth is these plugs offer little protection from freeze damage, and their primary purpose is to provide a method to remove the sand from the water jacket after the engine is cast in metal at the foundry.

When the engine is machined and assembled at the factory Frost Plugs are installed in the water jacket to seal it up. Over time the plugs begin to corrode and after years of service they will fail and begin to leak. Depending on the car these can be easy or difficult to replace while the engine is in the car. And in many cases the engine may need to be pulled to properly access the plugs for replacement. There are two common types of frost plugs, disc style and cup style. Although there are other styles including screw-in and rubber, these are less common on classic cars.

We join Mark Simpson as he works to bring our project 1931 DeSoto out of a 30-year long-term storage. After completing his initial inspection of the car there was little doubt the frost plug needed replacement. Mark demonstrates the correct process to remove the old frost plug, clean and prepare the block for the new plug then finally installing the new frost plug.

This process also gives him an inside look into the car’s cooling jacket as dirt and corrosion are prone to accumulate there and may indicate the need to further flush the cooling system.