mattbrown, et al.......after taking a torch to a frame...regardless of material, make SURE to have the threads chased with a chasing tool that's proper to the shell thread pitch.
one other very good bit of advice to pass along. stop using plumbers tape, teflon tape, lithium or silicone based greases, along with marine greases for bottom bracket shell threads and bottom bracket cup threads.
these sorts of products break down very fast and can purge easily on install.
instead, use anti seize. it's various powdered metals mixed with an oil that is designed to keep parts from seizing. it's water proof and won't purge like grease.....AND has a much higher temp rating than even the most expensive greases out there...even the one used for trucks/lorries.
i've had to use up to 4 and 6 foot cheater bars to get out seized cups....and every time i get them off....i always find something from the above list.
if you've been using grease since the paleozoic era and it's always worked, fine. but trust me when i say this, i've seen more ruined frames that could have been saved if anti seize was used.
