I've had good luck with an impact driver (the kind you hit with a hammer) and a fresh tip on the wrench. Sometimes touching the end of the allen wrench on a grinder to give it fresh-sharp edges (grind off a mm or so of the end) will help to get into the last good metal in the bottom of the head, but be sure to just touch the grinder (or belt sander) or you'll heat the allen wrench and soften /anneal the tool.

- Jeff Miller
190DT

Hi
A spiral flute screw extractor worked perfect to get it out (M103 waterpump). Torx did not work in this case.
http://www.stahlwille-online.de/pictures/all/900_fot_012.jpg
Good luck, Bruno