Bolts sometimes rust into place on car engines, electric motors or outdoor machinery like lawn mowers. When a bolt that threads into a solid piece like an engine clock breaks, the broken portion will need to be extracted. Two basic types of broken bolt extraction tools exist. The first is a square, tapered rod of hardened steel that has grooves cut into the corners; the second is a drill bit cut for counter-clockwise drilling.
In addition to the extraction tool, you need safety glasses, a center punch, hammer, electric drill or drill press, a drill bit that is one-half to three-quarters the diameter of the bolt (type 2 extractors only), wrench or tap handle that fits the extractor (type 2 extractors only), cooling oil, and a bolt extractor that is one-half to three-quarters the diameter of the bolt, vise or tie-down.
Secure the item with the broken bolt firmly in a vise or something similar, to ensure it does not move. Wearing eye protection, use the hammer and center punch to make a firm indentation at the very center of the broken bolt. This is to allow the drill bit to start without vibration and breakage.
Note that for any bit size above 1/4 inch (6 mm), you should drill a pilot hole with a smaller bit. This is also true of the type 2 bolt extractor. Drill slowly, using the cooling oil liberally to keep from dulling the bit by overheating it. Hold the drill as straight as possible, going into the bolt hole to avoid going off-center and drilling into the bolt hole threads.
Type 1 Extractor
The type 1 extractor is a square tapered rod of hardened steel that has grooves cut into the corners. This type is tapped into a hole drilled into the broken bolt, then turned with a wrench or tap handle. After drilling the broken bolt, place the correct size extractor into the bolt hole. The proper size should fit halfway into the depth of the bolt. Tap gently, straight down on the head of extractor, to set the extractor in the bolt. Place the wrench or tap handle on the extractor and turn gently counter-clockwise. If the extractor slips inside the broken bolt, remove the wrench and tap again. When it has set properly, the extractor will unscrew the remains of the bolt.
Type 2 Extractor
The Type 2 drill bit is cut for counter-clockwise drilling. This type is easier to operate, but less likely to remove a cross-threaded bolt. With the type 2 extractor, the drill must be operated in reverse. As more of the extractor bit makes contact with the interior of the bolt, it will grab and unscrew the bolt, with the broken part remaining on the extractor. Use pliers to gently unscrew the broken bolt from the extractor. Be extremely careful when doing this; misuse can cause injury.
After removal of the broken bolt, inspect both the bolt and hole for the cause of the problem. If the bolt was cross-threaded--meaning that the bolts' threads cut across threads in the block or work piece, then further repairs may be necessary. The threads for both the bolt and the block will be crushed and distorted rather than the sharp "V" shape of a new thread. If rust and corrosion were the cause, the hole should be tapped out and cleaned with lubricating oil to prevent a repeat of the break.
- Photo Credit bolts in the pile image by Dmitri MIkitenko from Fotolia.com
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