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Mucho PB Blaster/WD-40 over a couple of days with many attempts to twist it loose during the same time period. I've also used one of those newer strap wrenches to break one loose. Once you do get it thawed out, pull it out of the hole and coat the thing with a heavy grease or anti-seize so it don't happen again.
Go to the Garage and Workshop forum and click on the first thread "Read First". Scroll down to number five "The Candle Trick". Worth a shot if all else has failed. Works good for froze and broke bolts. Don't see why it wouldn't work for this too. Might toast the gasket or o-ring, but hey, they're cheap.
I have honestly never heard of pouring coke on anything but battery terminals or windshields. I will have to try that. Does it work on busting rust, or just on frozen parts?