Here's my strong vote for "heat and quench 3 times".
I am sure you got the right idea from all the previous posts on this
subject, but you need an OA torch with a really small tip, say a #1, in
order to heat just the set screw. Get it glowing and have a squirt bottle
of water handy. Once the set screw is glowing, squirt water on it until it
is cool to the touch. Do this 3 times. Then, gently remove the set screw.
A year or so ago I was faced with a couple dozen different kinds of
stuck/broken off bolts in an Onan genset and I asked for help everyplace I
could think of. The above was one of the suggestions I received late in the
game and hadn't tried it myself until a few weeks ago. A guy brought in a
front wheel drive car hub for bearing replacement. It had a sheet metal
guard held on by 3 10-32 (or metric equivalent) screws that had been there
since 1995. None of my normal stuff worked so I tried the above. It worked
3 out of 3 times. I am a convert.
But, you have to be able to really focus the heat, I think.
Note: one of the reasons you can heat the screw alone is that the
rust/corrosion between the screw and the hub acts as an insulator so the
screw heats faster than the hub (the hub having more mass). But, if you use
too broad a flame, or a torch with a much lower flame temperature(think
hardware store propane torch) the set screw will take longer to get glowing
and the hub will be closer behind in temperature, so the set screw won't be
crushing the rust as much.

See:
http://www.spaco.org/MachineShop/StuckFasteners.html
if you are interested in a range of solutions to this kind of problem.
Long time lurkers:
I haven't added to this page since Jan 23, 2010, when I posted its existence
here.

Pete Stanaitis
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