Allen bolt/screw head decapitation. Removal help please

[Cycling Cat]

Hi all
Internal thread on mountain bike arching bracket over front wheel for securing m/guard and reflector bracket.

The head rounded off two allen keys before I used hardened steel allen key, but this only decapitated bolt/screw head.

Any tips for getting stem out please? If drilling, are there special drill bits because mine don't make much of an impact?

Thanks in anticipation
C.C.

[531colin]

Its corroded in, then.
If you can get to the end of the bolt (rather than the sheared head end) I would try a cobalt bit eg http://www.toolstation.com/shop/p75149?searchstr=cobalt%20biuts.
maybe a 3mm bit to drill a 5mm machine screw? Use a battery drill, slow speed, high pressure, keep the drill bit cutting, as soon as it skates round you will simply polish the edge off the bit.
Drilling from the end will tend to unscrew the bolt, with associated heat from drilling friction.
If you can't get to the end, and have to drill from the head, then left hand drill bits can be useful, as they then tend to unscrew the bolt. again, battery drill, anti clockwise, slow speed high pressure. https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=left%20hand%20drill%20bits%20uk
I have never seen cobalt left hand drill bits, but you never know......

[rmurphy195]

Is the opposite end of the bolt hole open?If so, then using the drilling method may cause the remainder of the bolt to srew itswlef through and out of the open end, partially at least. Another alternative is to use a screw extractor of suitable size, I used these to remove the screws holding a Honda bike gearbox together many years ago. http://www.toolstation.com/shop/Power+T ... Set/p51010

Once you've drille a hole (carefuly!) down the length of the remaining bit of bolt, these screw in anti-clockwise, cutting theier own thread, then unscrwing the reminder of the bolt.

Failing all this - I think you are talking about the hole in the middle of the arch, at the top? If so, and it isn't a "blind" hole, you could always drill the hole out completely and then simply use a nut and bolt.

[Airsporter1st]

Just type "left hand cobalt drill bits" into ebay search field - loads available. Don't forget to use some form of cutting fluid.

As has been said, the advantage of left hand bits is the tendency to unscrew the remainder of the bolt whilst drilling.

[cycleruk]

It sounds like the bolt is corroded in the hole?
(Or it has "bottomed out" in the hole).
If corroded in then centre pop the bolt a few times which may help break the corrosion.
I doubt that a screw extractor would work in this case especially as they are very brittle at this small size and liable to break.
+1 for trying the suggestions in above posts.