Few events in the world of farm equipment repairs inspire profanity more than a broken bolt or stud. Yes, it's a challenge to remove a broken bolt, but with patience and the proper tools it's possible to remove it without resorting to colorful vocabulary.
If time allows, first soak the broken bolt or stud with penetrating oil and let it sit overnight. Given time, penetrating oils live up to their name and insinuate themselves deep into the threads. The extra lubrication is beneficial for any of the next steps.
On the way out. The following steps will depend on the size of the broken bolt, its location and the availability of specialty tools.
If the bolt's shank is ½" or larger, it's broken flush with the surface and it's in an easily accessed location, careful taps with a small sharp center punch can sometimes rotate the bolt out of its hole. Position the center punch's point close to the bolt's outside edge (but away from threads), and tap it so the bolt turns itself out. About 30% of big bolts broken close to the surface can be coaxed out of their holes this way.
Small and large bolts broken off below the surface generally require drilling and a bolt extractor. Some bolt extractor kits include left-handed drill bits. The vibration, heat and
reverse rotation of left-handed bits often remove broken bolts without a need for the bolt extractor.
Whether left- or right-handed drill bits are necessary, the key to drilling out a broken bolt is to precisely mark and center-punch the center of the bolt before drilling. After punching, drill deep into the bolt with a small-diameter bit, then use larger drill bits until the hole is at least two-thirds the diameter of the bolt.
After drilling the broken bolt, insert a bolt extractor. Economy-grade bolt extractors with square straight-fluted edges that have to be driven into the hole in the broken bolt run the risk of wedging the broken bolt tighter. Left-hand-spiraled bolt extractors, if tapped on their head with a hammer while twisted counterclockwise, will grip and
remove bolts without making them tighter.
If the first try with a bolt extractor doesn't work, it's time to turn up the heat.
Remove the bolt extractor and use an acetylene torch to heat the metal surrounding the bolt hole. Heat the metal as quickly and as much as possible without transferring heat into the broken bolt. The goal is to expand the surrounding metal away from the broken bolt, reinsert the bolt extractor and twist out the bolt before it absorbs too much heat and swells.
If the bolt still refuses to budge, cool it with penetrating oil. This serves two purposes: it shrinks the bolt and draws the lubricant into the threads. Be prepared for acrid smoke when the oil hits the hot metal. It may take several heating and cooling cycles to convince the bolt to release.
If the bolt still refuses to budge, use a series of ever-larger drill bits to enlarge the hole until it is slightly smaller than the bolt. When the bolt is a thin shell within the bolt hole, use a sharpened center punch to peel it away from the threads.
If the offending bolt is larger than ½" in diameter, stuck in a hole that's drilled through a piece of metal and there's open space on the back side, pull out an acetylene torch. Heat the broken bolt until it's yellow-hot from one end to the other, then blast a hole completely through the bolt. Be aware that even a slight twitch of the hand can veer the torch's cutting flame into "good" metal surrounding the bolt.
The replacement. Whether the broken piece comes out easily or with difficulty, use a thread chaser or tap to clean the threads in the hole before installing a new bolt. If the bolt hole's threads have been irreparably damaged, drill the hole larger and rethread it or install a Heli-Coil insert. Follow directions and the repaired bolt hole will be as secure as the original hole.
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