Core plugs, sometimes wrongly called freeze plugs or frost plugs, are plug

Plug

Plug may refer to:* Plug for the plughole in the bathtub, washbasin and sink* Plug , a short lived British comic that ran from 1977 until 1979, when it merged with The Beezer, which featured a character from The Bash Street Kids called Plug as it's main star.* Plug , a family of fishing lures*...

s that fill the core

Core (manufacturing)

A core is a device used in casting and molding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting, but are also used in injection molding.An intriguing example...

 holes found on water-cooled internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

s.

The sand casting

Sand casting

Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material.It is relatively cheap and sufficiently refractory even for steel foundry use. A suitable bonding agent is mixed or occurs with the sand...

 cores are used to form the internal cavities in the engine block or cylinder head

Cylinder head

In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...

(s), usually forming the coolant passage

Engine cooling

Internal combustion engine cooling refers to the cooling of an internal combustion engine, typically using either air or a liquid.- Overview :...

s. The traditional plug is a thin, domed, disc of ferrous metal which is pressed into a machined hole in the casting. Alternatively a non-ferrous metal such as brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 offers improved corrosion prevention. In some high-performance engines the core plugs are large diameter pipe plugs.

Core plugs can often be a source of troublesome leaks as a result of internal cooling system corrosion. Ease of replacement depends on accessibility. 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. Expanding rubber plugs are available as replacements when access is a problem.

Freeze plug

A true freeze plug is an expansion plug located in the side of an engine block that is supposed to protect the block against freeze damage. Water expands when it turns to ice, and if the coolant does not have enough antifreeze

Antifreeze

Antifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....

 protection it can freeze and crack the engine block. The freeze plugs (there are usually several) are supposed to pop out under such conditions to relieve the pressure on the block.

A variety of block heater

Block heater

A block heater warms an engine to ease and speed starting and vehicle warmup in cold weather. The most common type is an electric heating element connected through a power cord often routed through the vehicle's grille. The block heater may replace one of the engine's core plugs, or may be...

called a "freeze plug heater" can be installed, replacing the freeze plugs, to warm the engine before start up.

Welch plug

The Welch plug was originally designed by the Welch Brothers of the Welch Motor Car Company of Pontiac Michigan in the early 1900s.

"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 become the sole manufacturer of the Welch plug for the life of the patent." The Welch plug being the domed disk which is fitted against a shoulder in the core hole and then the dome struck with a hammer to collapse the dome and expand the disk to seal the core. Other core plugs are a dish design when pressed into the casting hole the tapered sides form the seal. These core plugs do not require the shoulder inside and are the principal design used in modern engines.