If coolant hasn't been changed on schedule Core Plugs(freeze plugs) will often rust from the inside out and start to leak.
These holes in the block are from when it was cast, hot metal poured into sand mold.
The holes allow the sand to be removed after the "core" cools down.
No, they are not threaded, a plug of the correct size is hammered/pressed into the hole at the factory, to seal the block for coolant.
If one of these plugs is leaking then it will need to be removed, you can often make the hole bigger and use a screwdriver or pry bar to bend it out.
You can replace it with a Rubber expansion plug of the correct size.
The rubber ones are easier to install because you can use a wrench to tighten them in place instead of a hammer.
Factory uses the metal expansion plugs for one reason only........they are cheaper, and in most cases will last the life of the engine.
So it isn't a case of which is "better" it is which is easier to use in your driveway.
Any auto parts store will have the rubber expansion plugs.
And just FYI, some of the "freeze plugs" will pop out if engine freezes, but that would NOT save the block from being cracked if it froze solid, lol, popular myth on why those holes are there.
Last edited by RonD; 01-03-2015 at 11:34 AM.