Do enough work on automobiles or other machinery, and it’s bound to happen: the head of a bolt shears off, leaving it stuck in the assembly without straightforward means of removal.
This happens often enough that tools and techniques for removal are well-developed. Following the methods below, a stuck, broken bolt can be removed.
When the bolt is secured by a nut:
This is the true “bolted joint”; a bolt or cap screw in a threaded hole is technically known as a “screw joint”.
When one cannot simply slide the bolt out of the other side of the joint, it is usually much simpler to remove the nut than to hold the nut in place and remove a broken bolt. Specialty tools known as “nut splitters” drive a wedge into a nut using an hydraulic press and crack it into two or more pieces. These are widely available from Sears, Harbor Freight, Amazon.com, Snap-On, and many other tool suppliers. Follow the instructions provided by the tool manufacturer, and be sure to wear eye protection.
If the workspace is so tight that a nut splitter will not fit, a cutting disk in a rotary tool or die grinder can be used instead to cut away the nut. A hammer and cold chisel can sometimes also be used to break the bolt below the nut or to crack the nut. In tightest spaces, wrap electrical tape around a hacksaw blade to create a makeshift handle and cut the bolt under the nut by sawing. With no tension on the blade, this will take a long time, but it works.
When the bolt is in a threaded, possibly blind hole in the assembly:
A bolt or cap screw in a screw joint, with no corresponding nut, is considerably more difficult to remove, but a common specialty bit known as a “broken bolt extractor”, “screw extractor”, or “easy-out” makes it possible with no specialized equipment.
The broken bolt extractor is nearly useless on its own. Other tools needed to complete the job are a T-handle to hold the extractor, an electric drill (preferably reversible), and drill bits hard enough to make a pilot hole in the bolt to be removed (preferably left-hand drill bits). A blowtorch, a center punch and a rotary tool similar to that made by Dremel or Foredom will make the job easier.
Step-by-step instructions are as follows:
(1) If the bolt is hardened or otherwise difficult to drill, anneal it by heating it with the torch, to red-hot temperature if possible, and allowing it to slowly cool.
(2) If the accessible surface of the bolt is uneven or convex, make a symmetric, concave depression in it using the rotary tool.
(3) As accurately as possible, make a punch at the center of the broken bolt.
(4) Using a small, hard, sharp bit, drill a pilot hole in the broken bolt.
(5) Work your way up, one bit at a time, to a pilot hole size large enough to accommodate your extractor. Drill slowly and steadily, using cutting oil if necessary.
(6) If you previously annealed the bolt, now harden it by heating it to red-hot and quickly quenching it with water.
(7) Using the T-handle (not the power drill!) screw the extractor into the pilot hole.
(8) With steady torque and no impact whatsoever, twist the broken bolt out of its hole using the T-handle and extractor.
(9) If necessary, clean up the outer threads in the workpiece using a tap, or repair them with a helicoil kit or plastic thread repair compound.
Some extractors on the market are designed for use with power drills. The procedure is similar, but follow the package instructions instead of using a T-handle.
Alden Tool now sells a “Drill-Out” broken bolt extractor kit, claiming that it is self-centering and allows bolts to be extracted using a single tool which first drills a pilot hole, then is adjusted to engage the broken bolt. Follow directions on the package. Centering these new “drill-out” extractors depends strongly on matching the size of the bolt; be sure to use the correct size for your application. I have no direct experience with this product; your success may vary.
If a portion of the bolt protrudes from the assembly:
Try Vise-Grips or similar locking pliers. If that does not work, plug weld a nut to the protruding end of the bolt and use penetrating lubricant, a socket and a breaker bar for removal.
If the bolt is iron or steel and the housing or assembly is aluminum:
A steel or iron bolt in an aluminum housing is particularly easy to remove by chemical means, provided that the assembly can be immersed in a bucket or tub, preferably one that can be heated.
Dissolve a quarter cup of aluminum alum in water and immerse the part. Heat to just below boiling if possible, otherwise, wait several days. The ferrous metal will be corroded by the alum solution, but the aluminum will be left intact.
If all else fails:
Look in the telephone book for machine shops that can remove broken drill bits or taps. Any shop that can remove a tap (a difficult task!) can remove a bolt. If these services aren’t explicitly advertised, look for “electrical discharge machining” or EDM. Often this can be done with no damage to the threads. If this service is not available in your area, search the Web, as quite a few shops are advertising services on shipped parts or assemblies.
These services can also remove stuck or broken “easy-out” broken bolt extractors, which are as hard as taps and cannot be drilled, although they can sometimes be ground out with a small diamond burr in a Dremel or similar rotary tool. Before using a broken bolt extractor, keep in mind that sending the part off to an EDM shop is “plan B”.
To avoid breaking bolts:
As the old saying goes: an ounce of prevention is worth a pound of cure.
Where there is little to no threat of a fastener “backing out” due to torque and vibration, apply a thin coating of nickel colloid “anti-seize” lubricant to the threads prior to assembly, especially for screw joints in which the screws are different material than the outer threads, e.g. steel cap screws in an aluminum engine block. Where this is not appropriate, use a thread-locking compound such as Loctite to keep moisture out of the threads.
Treat hardened bolts like drill bits or taps; avoid impact. When designing a piece of equipment, be sure that stresses on bolts are within specification. Tensile strength of bolts of given diameters and grades are well-tabulated and available from most fastener suppliers. Shear strength is not as often reported. A common “rule of thumb” is that shear strength is 0.6 times the tensile strength.
Most importantly, ensure that all joints are properly tightened, never re-use a fastener that has been heated with a blowtorch, and be sure that all replacement fasteners are of correct grade for the application.