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Posted by ChrisCase [63.199.243.144] on Monday, December 27, 2010 at 18:25:02 :
In Reply to: Best penetrating oil posted by Ed Bateman [75.140.24.232] on Monday, December 27, 2010 at 00:19:39 :
If I'm not concerned about the effects of heat, like loss of temper in a bolt I will be discarding, I heat the nut red hot. The nut expands when heated, making the hole bigger (the circunmference expands more than the diameter shrinks).
If the nut part is inaccesible, I heat the screw/bolt. See, rust is not just iron and oxygen, hydrogen is in there too. That is why water makes thing rust faster. On heating the red rust releases some of the oxygen with some hydrogen, making water (again), but also making the red rust into black iron oxide. The black iron is smaller than the red rust ( about 20%), so we end up with less rust in the holes to grab the screw. I believe it takes 760° F for the reaction to take place. That is less than red not, but I don't have any way to measure the temp, so red it is.
If heating the screw, let it cool and shrink back down in size, then lube and unscrew. If heating the nut, go ahead and unscrew while still hot, the nut is loosest then.
I think I posted this before, an archive search could turn up better details.