I just looked, and the Craftsman extractors I referenced go down to a #
5 screw. It had a reference for a size smaller than #6, and I thought
it was a #4, but alas.

Bud


Charles wrote:
Lots of good advice given but there's one problem that hasn't been
addressed. That's keeping the drill bit in the broken screw and not
letting it wander off into the spar. if the screw has been broken off
under flush there's not an easy way of dressing up the surface for a
good start. To keep from really messing up the job you may want to make
a drill guide to keep the bit aligned and going straight into the
broken screw.
A piece of 1/2" square steel stock about 8" long from any hardware
store will do. Set it up in a drill ress and drill a hole the size of
the bit you want to use through one side directly through. Set the bar
on the spar and line up the hole over the broken screw and then clamp
the bar down securely to the spar flange. Now you can drill through the
bar into the broken screw with far less chance of needing one of those
special figure eight shaped screws to cover the mess you made.
Any good quality SHARP drill bit will do the job with the correct
technique. For drilling out steel screws, use a slower RPM and more
pressure than for drilling aluminum. Start with a couple of light taps
at high RPM to ensure the drill gets a good center on the broken screw
shank or use a small center punch that will fit through the guide.
Using cutting oil is a good idea.
I don't think the Craftsman extractors are small enough for a #4
screw. I would go with a left handed bit, mouse milk, and take my time.