Hey guys, I'm working on a bike (threaded headset) where I'm swapping out the stem. Yesterday, the stem was loose enough that I could easily rotate it independently of the fork, and remove it with some wiggling. I tightened it, and then today, when I went to loosen it and remove it, the stem is totally, irremovably stuck. If I loosen the stem bolt, it just rises; taps with a mallet don't budge it. If I remove the stem bolt completely, the stem still doesn't budge. How could this possibly happen overnight? I'm up for trying to put oil, ammonia, in it, etc, but it seems like it must be another problem, because it's not like the stem corroded in place overnight. Any ideas? Thanks,

-Colin