When an engine block is manufactured, a special sand is molded to the shape of the coolant passages in the engine block. This sand sculpture is positioned inside a mold and molten iron or aluminum is poured to form the engine block. When the casting is cooled, the sand is loosened and removed through holes in the engine block casting leaving the passages that the coolant flows through. Obviously, if we don’t plug up these holes, the coolant will pour right out.
Plugging these holes is the job of the freeze-out plug. These plugs are steel discs or cups that are press fit in the holes in the side of the engine block and normally last the life of the engine with no problems. But there is a reason they are called freeze-out plugs. In the early days, many people used plain water in their engines, usually after replacing a burst hose or other cooling system repair. “It is summer and I will replace the water with antifreeze when the weather starts turning”.
Needless to say, people are forgetful and many a motor suffered the fate of the water freezing inside the block. Often, when this happened the pressure of the water freezing and expanding forced the freeze-out plugs to pop out, relieving the pressure and saving the engine block from cracking. (although, just as often the engine cracked anyway). Another reason for these plugs to fail was the fact that they were made of steel and would easily rust through if the vehicle owner was careless about maintaining the cooling system. Antifreeze has rust inhibitors in the formula to prevent this from happening, but those chemicals would lose their effect after 3 years, which is why antifreeze needs to be changed periodically. The fact that some people left plain water in their engines greatly accelerated the rusting of these freeze plugs.
When a freeze plug becomes so rusty that it perforates, you have a coolant leak that must be repaired by replacing the rusted out freeze plug with a new one. This job ranges from fairly easy to extremely difficult depending on the location of the affected freeze plug. Freeze plugs are located on the sides of the engine, usually 3 or 4 per side. There are also freeze plugs on the back of the engine on some models and also on the heads.
As long as you are good about maintaining the cooling system, you need never worry about these plugs failing on modern vehicles