When it comes to rounded-out allen bolts and screws, the first thing I try is a torx bit (the kind with a 1/4" hex drive). They tend to have a slight taper on their splines, which really helps bite into the corners when you gently hammer the bit in. Generally works a treat above 3mm.
For smaller stuff, like the bastard 2mm grub screws that hold the brake pivot pin in STI levers, I came up with an alternative: get some foil (start with more than enough) and tightly wrap it around the key, and twist off the excess where it overlaps the end. This part gives you the ability to jam as much foil as will fit in, but you shouldnât be able to at first; after the first attempt, remove a little foil and try again. Once you have the key in place, surrounded by the maximum amount of foil that will fit, itâs surprisingly effective. Of course, donât forget the penetrant lube.
