BROKEN BOLT REMOVAL - the easy way. Over the years I have seen and heard dozens of suggestions for tackling this problem - some more successful than others. One common approach is to drill a hole down the centre of the broken shank of the bolt and use a popular little hand tool commonly known as an “Easy-out” with a left hand self-tapping thread to try and unscrew the broken shank. The problem with this method is that “Easy-outs” are a tapered shank tool, so that as they are drawn into the broken bolt shank during the attempted removal process, they tend to expand the damaged bolt to an even tighter fit in the parent hole, usually resulting only in the broken bolt becoming even more difficult to remove or even worse - having the broken shank of an “Easy-out” jammed into it.
A far more successful method is to identify the largest allen key that you can accommodate within the thread root diameter of the broken bolt. Measure the outside diameter of the Allen key (i.e. over the points, not across the flats of the hex), then select the next drill bit below that size (but which is larger than the diameter of the allen key measured across the flats). Use a fine toothed Hacksaw or grinding disc to then cut a straight piece (as long as possible) from the shank of the allen key.
Now drill a hole into the centre of the broken bolt shank - the hole should be about the same depth as the outside diameter of the broken bolt. A high speed steel drill bit will drill a hole in most normal materials, but if you are dealing with a very high tensile bolt, you may need a cobalt or even a solid carbide drill bit. The idea is then (making sure that you are wearing safety glasses and appropriate protective clothing), to use a ball-pein hammer to drive the allen key shank into the hole you have drilled.
Hammering the allen key shank into the hole you have drilled in the bolt will cause it to broach its own splines into the inside of the hole which will then allow the broken bolt to be easily removed with the aid of an appropriately sized socket and driver fitted to the protruding end of the allen key shank. Cheap, easy and it works!
Naturally,although the above information is given in good faith, since we have no control over the process, quality of materials used or operator skill, sole responsibility for the use of this information rests with the user