Originally Posted by BostonFixed
I have not personally tried this method on a cog, but It worked for me to get an old BB off a POS frame. When I used to work in a sketchy LBS, we also used this method to get parts off of old junker frames. Not specifically on cogs, but seatposts, bbs, etc.
I think that the alu will expand faster, but the expansion/cooling of the cog/threads may break the "seal"...
Makes some sense on steel + steel stuff (old frames + seat posts and bbs/ect) to try and take it off while hot. I didn't really think about the heat/expansion/cooling of the cog/threads breaking the crusted on, rusted on, threaded on, gunked on seal that may have formed...good idea.