Classic Mustangs (Tech)Technical discussions about the Mustangs of yester-year.
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"Freeze plug" is a misnomer. Since the "freeze plugs" have nothing to do with protecting the engine from freezing, it's entirely possible that your block could be destroyed by ice, with or without popping plugs.
These devices are actually core plugs, and the holes are the means by which the sand cores are supported when the block is cast.
Amateur restorer. (Well, once in a while I have been paid for it)
Just replaced two leaking freeze plugs and while discussing the very topic 2+2 mentioned the person I was talking to said he has never seen a "freeze plug" pop when the coolant froze. So yes you could cause severe damage to the block and a freeze plug would never show it.
What I would do is check the coolant with a tester just to see what temp it was good for and ease my mind that the plug just rotted and the block probably never froze. If you can't get the engine out to properly replace the plug right now go an get the emergency rubber plug.
Even with the plug behind the motor mount you can jack the engine up with a jack and 2x4 supporting the wide part of the oil pan. Undo the bolts, jack the engine up remove the top mount and get to the plug. Not fun, but doable. I don't know how long i would trust the plug. I just put one in my 67 and have heard people say they drove around forever with them, but i have read that they are just an emergency fix.
I popped two of them last winter. Both behind the motor mount on each side. I jacked the engine up, removed the motor mount and put them back in. Nary a problem since. I think they just weren't in very good. They went in pretty easy.
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