Well, as per above I managed to mess it up. I have tried drilling, but would be grateful for some numpy advice on how this is done. My good SDS drill does not have bits small enough, but I am sure I can get them online.

This was all driven by my desire to get rid of the annoying rotor squeal on the rear of my hard tail. I have changed the brakes, but retained The old 160 mm disc. I made sure it was clean with disc brake cleaner before remounting the new disc brakes and wheel, to a coif contamination.

Sequels like a banshee and sometimes vibrates prior to getting. A squeal on.

I have a spare 180 and spare 160 disc, so could just try a larger diameter on the rear.

I usually put a slot in it with the angle grinder and the use a big flat blade screwdriver. Since we bought a Park Tool t driver Ive not had to resort to force.

A nice, wide drill to get the head off. Then undo the other bolts. The rotor will then come off leaving a thin stub of bolt sticking out of the hub. Grab this with molgrips, or stick it in a vice and turn the wheel.

Or drill the head off and leave it, one bolt missing on the Brake disk won’t kill you.

this way has worked for me on more than one occasion, doh.
if its only one bolt, lift the opposite side of the rotor to the stuck bolt, and rotate the rotor just a little till it loosens the bolt, you may be able to use the the allen key now, if not just keep turning the rotor while lifting it a little to keep the bolt under tension. if you are careful the rotor shouldnt bend.
you do need to lift the rotor above the slots, if any on the wheel/hub mount.
i just had to do this myself. top tip imho, throw rotor bolts away after used once, and me personally i take the blue loctite off, they havent come undone yet

I’d slot it with a dremel.

I recently bought a cheap set of wheels where the sellers LBS has made a right old mess of getting the two bolts out leaving very little to work with.

One I cut the back of the bolt off, drilled into the centre with a small drill bit and this was enough to rotate it through and take it out the back.

The second was completely screwed, when they’d drilled it out I think the drill had tracked off, got hot and then melted. Had to fix that with a helicoil kit.

Slot head with dremel cutting disc is my preferred method.
That and tighten the two bolts either side of the busted one to take a bit of pressure off

I rounded a rotor bolt last night. The method Lester describes above worked a treat.

[/quote]I recently bought a cheap set of wheels where the sellers LBS has made a right old mess of getting the two bolts out leaving very little to work with.

ha ha , a likely story from the seller .

Maybe. But the price was worth it.

The correct method is to grind the head off, drill a wee pilot hole and then use a tap wrench and thread extractor.

I’ve used a battery drill, clamped the bolt head in the chuck & wound it out on a low speed setting, done this on a number of damaged bolts/screws, easier on socket cap bolt heads.

Molegrips have always done the job for me

Stud extractors may technically be the right answer, but I dislike using them in such small sizes. Far too easy to snap it off and then add a chunk of toolsteel in the bolt cap to you woes. For something like a rotor bolt I’d probs just drill the head off and then get molegrips or similar on the shaft once the rotor is clear… or possibly even a two-nut and wind solution.

My method is to put two hacksaw blades in the handle and use it to cut a slot in the bolt head then with a big screwdriver and plenty of pressure or an impact driver drive the screw out. Heat and penetrating fluid on the bolt as well.

Only if that doesn’t work go for the drilling methods

As ever you’ve got to think about why the original bolt stripped. Was it clumsiness, a bad tool? Or is it really, really stuck. That’s important because it’ll possibly be just as stuck after you take the head off.

I’ve had some good results with undoing the other 5 bolts then rotating the rotor. It doesn’t always achieve anything mind, the bolt may not move with the rotor. But you lose nothing if it fails- lots of these methods are pretty much one-shot.

Not on a disc bolt but I’ve just tried that clamping the screw head in a drill chuck. Bloody genius.

If its a Allen key head screw then you can hammer a torx bit into it which bites enough from experience to undo. If that doesn’t work then as above, slot the head with a dremmel or a hacksaw. Always worth leaving some plus gas or similar on it first. Drill out as a very last resort, as if it goes wrong, it goes very wrong!

Thanks for all the inputs on this. I purchased a Dremmel and ground the head off, removed the disc and then got mole grips around the remaining bolt. Job done.