Broken bolts and studs are fairly common occurrence when restoring or rebuilding an older vehicle.  Removing the remaining piece without damaging the threads, in that very difficult to replace part, can be quite a challenge.

The following are a number of methods that can be tried to help in this situation.

In all the following suggestions, success is greatly assisted if the offending bolt is first heated and allowed to cool.  This has the effect of breaking or weakening any bonding (be it locktite, gasket-goo or rust) between the part and the bolt.

1.  A small part of the broken stud is protruding from the surface

The following method works best if there is some bolt still sticking up where you can get a hold of it.  Heat the bolt only first, then let it cool naturally.  Heat the area around the bolt AND the bolt at the same time.  While the bolt and surrounding area are hot, get an ice cube and put on the bolt ONLY.  BE CAREFUL - WEAR GLOVES FOR PROTECTION.  Usually, if you can get a pair of vice-grips on the bolt, it will spin right out.  No drilling required.  Hard to believe, but it works.  Try it.

If you have access to a MIG or TIG welder, try welding the corner of a bit of scrap steel (say 1 x 1/8) to the protruding bit of the stud.  This does two things: the heat helps to loosen the stud in the hole, and the steel gives you a lever to turn.  With luck the remains will screw out and you won't need to worry about metal swarf near your engine.

Alternatively try welding a nut, one size smaller than the threaded portion, to the broken stud.  Weld the inside.  The heat from welding may break the corrosion bond, and the nut gives you something to grab and turn.

2.  Studs broken off flush with the surface or just proud

These usually require a hole to be drilled into the stud and an extracting tool such as an ezyout, used to unscrew the broken remnant.

What must be avoided is damaging the threads of the hole into which the stud is screwed.  This damage is usually the result of drilling into the stud off centre, causing either the drill or ezyout to damage the thread as it breaks through the side of the stud.

To avoid this situation one needs patience... and lots of patience.  In other words don't try and hurry the job.

Start with a file or grinder and make the exposed surface of the stud as flat as possible.  This allows the centre of the stud to be more easily determined, allows a good deep centre punch mark to be made and prevents the drill from wandering off centre.

A drill guide can easily be fabricated to ensure that the drill starts off in the centre of the stud and remains there.  Weld some locating pins on a piece of 1/8" thick flat steel, then drilled 2 holes in another piece of steel to match the locating pins and put that piece on top and drill a small hole through both pieces of steel.  Then drill a hole the same size as the stud hole on the bottom piece of steel and clamped it up against the part.  With the hole the same size as the stud you can centre it pretty accurately, put the second piece up (that is why the locating pins) and drill through it using the small hole that we know is dead centre to the bigger hole.

Then use the next size up drill bit (a reversible drill with a left hand bit, if you can get one, is best) and the stud should come out as you drill the larger hole.

If it still doesn't, try using an ezyout or gradually go up in drill size until you are in danger of cutting into the threads in the part.  Once the wall thickness of the stud is thin enough, it can be broken or peeled away from the hole with a fine centre punch and hammer or even a scribe.

3.  Stud broken off below the surface

For studs broken below the surface, turn a small bushing on the lathe that fits into the hole and drill a small hole in that bushing (while still in the lathe, to be sure it is dead centre) and use it as a guide to drill the broken stud.

If the stud is not far enough below the surface to utilize a guide, then use a Dremel type grinder so that you can grind the broken stud down so that it is well below the face of the flange.

Chances are that you won't get the hole dead centre; do the best you can.  Gradually go up in drill size until you just start cutting into the threads; inspect often with a mirror and flashlight (tip:  point the flashlight into the mirror).

4.  Using an EzyOut

There are two sorts:- straight fluted and twist fluted.  The straight ones are preferred, because all the force is used to turn the broken stud, rather than twisting the ezyout in further.  However the twist ones are more amendable to weird hole sizes.  Chances are that you won't get anywhere with the ezyout, but try it anyway.  Be careful with the ezyout.  If the bolt was stuck enough to break off, it's usually stuck enough to break an ezyout, and then you have BIG problems.

Heat the area with a torch, spray on some Penatrine or WD-40 and wait.  Repeat several times.  This may help it may not.  Try the ezyout again.  Repeat until you're frustrated and ready to try enlarging the hole, or, if you're terribly lucky, the blasted things comes out.  (It may take several days of this cycle to succeed.  If you're not in a hurry, this is the safest way.  The hotter the torch the better - try to get the part red hot.)

5.  EzyOut or Drill but breaks off inside stud

Bugger...

A process called EDM or Electric Discharge Machining can be used to burn an exotic alloy bolt or extracting tool out of a much weaker metal without damaging the weaker metal.  This is obviously not something that can be done in the average garage, it requires the part to be out of the car and is not cheap.

Bibliography credits
 Morris Register of Victoria, March 2005, Vol.28 No.6