In Norwegian these are called "Skrueuttrekker" or "Grisepikk" (Loosely translatable to:"Screw-undriver" or "Pigs dick", don't ask. I believe its titulated as a screw extractor. )
Idea is that you drill out center of the stubborn bolt, then screw on of these fellas into the hole and twist out the bolt.
Good thing about it: It can be done with the engine in the scooter. Bad thing: Drilling freehand in the centre of a bolt will be demanding. The extractor can also get stuck.
I think I'll give it a go. Soon.
