Fasteners such as studs, bolts and screws can fail for a number of reasons: over-tightening, metal fatigue or overstressing. Fasteners broken off above the surface are relatively easy to extract, compared to bolts or studs broken off inside a threaded hole. Using a bolt extractor to remove a broken fastener can be risky--a bolt extractor is hard and brittle; if too much force is used, it will break off inside the hole and become impossible to remove.
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Things you need
- Penetrating oil
- Oxyacetylene torch
- Hand grinder
- Center punch
- Ball peen hammer
- 1/16-inch twist drill
- Reversible power drill
- Bolt extractor set
- Set of left-handed drill bits
- Masking tape
- Two-handed tap wrench
Spray a liberal amount of penetrating oil onto the broken bolt and wait for 1/2 hour. Repeat two or three times.
Apply heat to the broken bolt and surrounding area with an oxyacetylene torch fitted with a medium size nozzle. Open the red acetylene valve and light the torch. Open the oxygen valve slowly until you have a smoky carburising flame. Pass the end of the flame over the area, depositing a layer of carbon soot on the surface.
Adjust the torch to a normal flame. Hold the torch nozzle 9 to 12 inches away, and direct the flame 3 or 4 inches to the side of the bolt. Keep the flame moving in circles around the bolt without aiming directly at the black carbon deposit. (When the metal heats up sufficiently, it will burn off the carbon.) Once all the carbon has disappeared, concentrate the flame on the bolt until it starts glowing. Turn the torch off and wait for the metal to cool. Apply more penetrating oil.
Grind the face of the fractured bolt flat with a hand grinder if the break protrudes above the surface. Position the point of a centre punch exactly in the centre of the broken bolt face. Deliver a light blow with a ball peen hammer.
Using the centre punch indentation as a starting point, drill a 1/16-inch pilot hole into the bolt with a reversible power drill. Hold the drill as close to a 90-degree angle to the surface as possible. Be careful and take your time--this is a crucial step in the extraction process.
Read the instructions that came with the extractor set. Select the correct size drill bit and extractor for the bolt you are working on.
Select a 1/8-inch drill bit from the set of left-handed drill bits. Measure the broken thread depth from a similar bolt with a ruler. Transfer the measurement to the drill bit, and mark the spot with a pencil. Wrap a piece of masking tape around the drill bit with the edge of the tape nearest the point of the drill bit touching the pencil mark. This will act as a drill depth guide.
Set the drill in reverse. Insert the 1/8-inch drill bit in the chuck and tighten. Drill into the pilot hole and continue straight through the broken bolt with the left-handed drill bit. Keep the drill at a right angle to the surface. Stop when the edge of the masking tape on the drill bit touches the end of the bolt.
Change to a left-handed drill bit two sizes up and repeat this process. Progressively enlarge the hole with increased drill sizes until the hole is close to the diameter of the extractor drill bit. (Following this procedure will make drilling easier and prevent the hole from wandering off centre.)
Adjust the drill to operate in a clockwise direction. Insert the extractor drill bit and drill the final pass. Fill the hole with penetrating oil. Wait 1/2 hour.
Insert the recommended left-handed spiral bolt extractor into the hole. Deliver a light blow to the end of the extractor with a ball peen hammer to set it firmly in the bolt. Clamp the square end of the extractor bit between the jaws of a two-handed tap wrench.
Apply firm anticlockwise pressure to the tap wrench. The extractor will bite into the bolt before forcing it to break loose. Keep turning the tap wrench anticlockwise to remove the broken bolt.
Tips and warnings
- A clockwise rotating drill bit will tend to tighten the broken bolt, whereas using left-handed drill bits will tend to loosen it.
- If the bolt broke off while trying to remove it, it might be frozen solidly in place. If you sense that the extractor might break while trying to extract the broken bolt, it would be wise to abandon your efforts rather than having to replace the component holding the broken bolt. Continue enlarging the hole, drill out the broken bolt, and insert a Helicoil thread adaptor to bring the thread back to its original size.
- Wear safety goggles when drilling and using a bolt extractor.
- Use leather welder's gloves when working with an oxyacetylene torch.
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