One of the aluminium base screws is stuck in my caravel which someone mounted the base on upside down. I can't get it out. Tapping with a hammer, penetrating oil, etc. have not yielded any success.

The paint is very beat so I am not against heating it with a torch, and if it did not have the little aluminium cup blocking access, I could probably grab it with vise-grips, but I assume there are not spares readily available.

Any tips? The screw head is not exactly stripped, but it's not in great shape either.

Having no idea about what this looks like I'll say this which might make no sense at all.

When all else fails, try a left handed drill just a bit bigger than the threads. It might catch and unscrew, that happens sometimes but if it doesn't and the base is thick enough there might be enough to grab with a vice grips.

Push down hard so it takes a big bite at first and has a chance to unscrew it. Better with a drill press so you can limit downward travel as you don't want to touch the base.

Ira

Finally I had success after some wrestling with it tonight.

Heating the base with a torch, penetrating oil, tapping at the head, using a small screw driver and a hammer to try to rotate the screw forcefully did nothing. I finally broke down and widened the aluminium cup into an oval shape using a roundhead rod and a small hammer. That allowed me to get a pair of vise-grips with ground down jaws around the head of the screw. After a few attempts at getting the pressure of the vise-grips just right, I mounted the them in a vise, and rotated the machine itself which broke the bond and allowed me to remove the screw by hand. Bottom half of the screw threads were coated in that white corrosion product you sometimes find on aluminium.

The aluminum cup is not in the best shape, but I'm pressure I'll be sure I'll be able to make it round again and it should clean up enough.