Core plug

A core plug that has been corroded from improper engine maintenance.

Core plugs, sometimes wrongly called freeze plugs or frost plugs, are plugs that fill the core holes found on water-cooled internal combustion engines.

The sand casting cores are used to form the internal cavities in the engine block or cylinder head(s), usually forming the coolant passages. 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 offers improved corrosion prevention. In some high-performance engines the core plugs are large diameter pipe plugs.^[1]

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 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 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."^[2] 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.

References

1. ^ Monroe, Tom (1996), Engine builder's handbook: inspection, machining, reconditioning, valvetrain assembly, blueprinting, degreeing cams, tools, engine assembly, HPBooks, p. 111, ISBN 9781557882455, http://books.google.com/books?id=p1zb-keWM74C&pg=PA111. 
2. ^ http://www.britishcarweek.org/welch_plug.html