Easiest method of removing a rounded Tensioner Pulley Bolt

As title says, I am an idiot

weld a nut to it if heat is no problem there.
Due to language and tech slang i am not sure what bolt this is and if there is any sim rings, gaskets or bearings near by that the heat will damage.

Heat will also help to loosen stuck bolts so welding a nut to it is often a great way to loosen bolts.

Grinding down bolt head to next dimension and use a 6 side socket as lambert says.

Metrinch sockets are designed for this purpose. There are other makes on the market. basically they don't have flats but are curved like a never ending roller coaster so grip on to worn bolt/nuts. I have a socket/combination set and am forever using it. You can buy separate individual sockets.

You mean this pulley?





There should be room to get one of those Irwin extractors in, or take the other bolt right out, get as much strain on it as you can, and try tapping the whole tensioner round anticlockwise to "help" it a bit, you could even try helping it with the hexagon key.

Don't tap the outside of the bearing, only the solid centre part!