When bolts or studs snap, they often do not leave a perfectly flat surface on which to start drilling.
This is a technique in which you can perfectly center a pilot hole for drilling out broken bolts and studs.
The idea is to drill out the center of the same size bolt as the one broken off in a hole, or as the same size as the stud sticking out.
As this video illustrates, you then proceed to use it as a drill guide:
You can make your own in a drill press:
Or what most people dont know, is that there are “vented” bolts and screws already available. They are created for the purposes of using bolts in vacuum sealed environments or in situations where grease or pressure is not desired below the bolt.
The same method can be used on studs by joining a drilled or vented bolt with the stud, using a nut, as such: