Sun Dec 31, 2023 10:45 am by cornbinder89
To expand on Nikki's comment about them being "core plugs" not "freeze plugs". That is 100% correct, you'll not find them on castings that have an opening to remove the core sand. That includes wet liner engine and ones with an open deck like the Detroit 8.2 and many imports. Castings with enclosed voids, need a way to get the core sand that form the void in the casting out, once the casting has cooled. Rough holes are left, than machined to take pressed in covers to seal.
Often you can see the remains of rods or "struts" that help hold the sand in place, but can not be removed, still in the head or block, they often become part of the cast wall at both ends.
Good casting is an art, and mold-makers are highly skilled. Take a cylinder head for example, making the mold for the intake and exh passages doesn't seam too hard, but making the coolant passage that must surround the other "voids" but not connect to them is a real art.