RRRrrreceived this from the Michigan Mob: With the regard to "expansion plugs" and "freeze plugs", there is no word description in any parts book from 1951 thru 1965 that uses this terminology. All references to these "plugs" are listed as "plugs, core hole". These "holes" are cast into the blocks and heads to remove the sand cores that remain in the block and heads after the cast iron is introduced into the engine mold. The only way to remove the sand cores inside the castings is thru these "core holes". The inner mold is to create the water passages inside the casting. Then when all sand material is removed, the "core holes" are machined to accept a "plug". There are two types of "plugs", early are dish type, later are cup type. Both are installed with a special tool at the factory for each type. "Expansion" and "freeze" plugs is terminology used by by the layman mechanic, which really is more understandable by everyone. George Riehl