Sand cores are used to form the internal cavities when the engine block or cylinder head(s) is cast. These cavities are usually the coolant passages. Holes are designed into the casting to support internal sand forms, and to facilitate the removal of the sand after the casting has cooled. Core plugs are usually thin metal cups press fitted into the casting holes, but may be made of rubber or other materials. In some high-performance engines the core plugs are large diameter cast metal threaded pipe plugs.
Core plugs can often be a source of leaks due to corrosion caused by cooling system water. Although modern antifreeze chemicals do not evaporate and may be considered "permanent", anti-corrosion additives gradually deplete and must be replenished. Failure to do this periodic maintenance accelerates corrosion of engine parts, and the thin metal core plugs are often the first components to start leaking.
Difficulty or ease of core plug replacement depends upon physical accessibility in a crowded engine compartment. In many cases the plug area will be difficult to reach, and using a mallet to perform maintenance or replacement will be nearly impossible without special facilities for partial or complete removal of the engine. Specialized copper or rubber replacement plugs are available which can be expanded by using a wrench when access is a problem, though engine removal may still be required in some cases.
The term Freeze plug is slang, the name of the press in block plugs is actually core plug. It is mistakenly thought that the purpose of these plugs is to be pushed out and save the block from cracking if the engine has water in it and it happens to freeze. This in nothing more than an urban legend or an old wives tale. The purpose of the plugs is to fill the holes that were made during the casting process. so the foundry could remove the core sand from the coolant passages. Saving the block from cracking in case of a freeze was never the manufacturer's intent for these plugs
The Welch plug, (misnomer: Welsh plug), is a thin, domed disc, of a metallic alloy, which is pressed, convex side out, into a casting hole and against an internal shoulder. Alternatively a non-ferrous metal such as brass offers improved corrosion prevention. When struck with a hammer, the dome collapses slightly, expanding it laterally to seal the hole. Other core plugs have a dish design, so that when pressed into the casting hole the tapered sides form the seal.
According to Nevin Hubbard of the M.D. Hubbard Spring Company, the Welch plug was originally designed in the 1900s by the Welch brothers at the Welch Motor Car Company of Pontiac, Michigan. Hubbard claims that "at that time core holes in the engine blocks were fitted with pipe plugs. During one of these run-ins a pipe plug backed out. In order to get back on the road one of the brothers drove a quarter or half dollar into the hole. From this they developed the Welch plug, some with the help of my great grandfather Martin Hubbard. They then patented the plug and the M.D. Hubbard Spring Company became the sole manufacturer of the Welch plug for the life of the patent."
A variety of block heaters called a "freeze plug heater" can be installed, replacing one of the core plugs. The electrical heater is used to warm the engine coolant (and therefore the engine) before start up.