Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use.
Core Plug
Metal disc which seals off the end of the core left in the cylinder block (and, sometimes, cylinder head) by the casting process. The core plug can also act as a crude form of safety device against frozen-up cooling systems, by allowing the freezing water to expand and force the core plug out of the block, thus preventing the forming ice from cracking the block.
