One method for removing damaged bolts involves bolt extractors with a reverse spiral flute that grabs the remains of the bolt head when the extractor is turned in a counterclockwise direction. Using this time of bolt extractor is fairly intuitive when you see the tool. Just choose an extractor slightly smaller than the head of the bolt you're removing and press down while turning the extractor counterclockwise. There are several different styles of bolt extractors. Some are made to work with a reversible ratchet and have a square hole to fit either a ½ inch or 3/8 inch drive ratchet. Other have a hexagonal head and are meant to be used with a box-end wrench. The disadvantage to the extractors meant to be used with a ratchet is that if a bolt protrudes through a badly worn nut more than a fraction of a few centimeters, the ratchet may not fit all the way into the extractor. My preference is for the extractor with a hexagonal head because it's easy to use with a box-end wrench.
Nut splitters sometimes come in handy but have limited use on motorcycles because of their bulk. Nut splitters generally have a wedge and an anvil and one of those that forces the two parts together when the bolt is tightened. Using a nut splitter involves placing the splitter over the damaged nut with the wedge against one flat and the anvil against the opposite flat. Slowly tightening the bolt presses the wedge into the flat until the nut splits. However, some nuts will split quite harmlessly while other fracture abruptly and send sharp metal fragments in several directions at once. I cover the nut splitter with a shop rag in hope of containing any shrapnel created by using this tool.
Another need trick I learned while working on old motorcycles: use a grinder or other cutting tool to create two new flats on opposite sides of a damaged nut or bolt head. After using this method on a bolt, you grab the two new flats with an adjustable wrench or locking pliers. When working with damaged nuts, the adjustable wrench may be the preferred tool because locking pliers can squeeze the nut onto the bolt and make removal more difficult. An alternative to the Easy Out is the power drill-out. The power drill-out combines a left-hand drill bit and an extractor in one tool and consists of a cutter and a collet on the same shaft. The cutter is used to drill a hole in the exact center of the bolt being extracted. The collet, which has a left-hand thread on its outside, grabs the bolt when it is threaded against the end of the bolt. If you buy a set of power drill0outs, follow the instructions that come with them and use plenty of cutting oil.
Another type of extractor is the multi-spline extractor. This type is not meant for working with splined shafts. It gets its name from the multiple splines that spiral down the working end. Multispline extractors have several advantages compared to other extractors. All 10 of the multispline extractors in the kit I own have a 12.7mm hexagonal head. The smaller sizes can be used to remove Torx head screws and bolts, as well as socket head screws and bolts. The gentler taper of the multispline extractor, compared to the more common bolt extractor, is less likely to cause the end of the bolt to flare out and become more firmly stuck.
To use a multispline extractor, first center-punch the broken bolt. Then drill a pilot hole using the size drill recommended in the chart that comes in the kit. Now lightly tap the spiral end of the extractor into the hole you just drilled and turn the extractor counterclockwise. The splines should grab the inside of the pilot hole and rotate the bolt out.
One more tip I picked up over the years is simple and inexpensive although it only works if the bolt is free to turn after the head has broken off. Find a sharp chisel with a width slightly less than the diameter of the headless bolt. Place the chisel against the bolt and hit it with a ball-peen hammer. Then look at the end of the bolt, and you should see a slot created by the hammer and chisel. You may be able to use this slot with a screwdriver to unscrew the bolt. This technique doesn't always work, but when it does, it may save you additional effort and expense. I once used this trick to remove a gearshift bolt from an old BMW motorcycle, saving the rider from more expensive repairs and downtime.
Now that we've discussed all these different ways to remove broken or damaged nuts, bolts, and screws, it's time to talk about ways to avoid damaging fasteners in the first place. The biggest source of harm is using the wrong tools.
Adjustable and open-end wrenches are notorious for rounding off the points on hexagonal nuts and bolts. Tool sales-people may tell you that a 12-point wrench can be used on six-point nuts and bolts, but what they might not tell you is that 12-point wrenches cause excess wear on six-point hardware. Torx drivers and Allen wrenches may look similar, but they are not interchangeable. People who try to remove Torx fasteners with an Allen wrench usually end up with a damaged screw that neither tool can turn. Phillips screws come in eight sizes, ranging from the large P4 down to the tiny P0000. Using the wrong size screwdriver can damage the screw head and the screwdriver.
Using the wrong type of bolt can also cause problems down the road. The physical characteristics of a bolt involve more than length, diameter, and threads per millimeter. Bolts have grades according to their strength. Look at different bolt heads and you may see lines which indicate the grade. If you try to save a few satang with an inexpensive hardware store bolt instead of the grade five or eight bolt you should be using, the bolt may break at the most inopportune time.
Did you know that chemical thread-lockers are generally color-coded to indicate their strength? I use purple threadlocker on smaller screws and either red or blue on larger bolts. Riders sometimes use locomotive-strength threadlockers when installing accessories on their motorcycles, then wonder why the screws won't come out the next time a repair or adjustment is needed. The bolt bin in my shop contains some OEM replacement bolts with what looks like red paint on the threads. That's not paint; it's threadlocker applied by the factory in the right quantity and in the right location for that bolt. Whenever I see that material on the thread, I don't add any more threadlocker.
Steel bolts that thread into aluminum castings are a special situation. Dissimilar metals sometimes react by seizing or galling and can be quite difficult to remove. A thin coat of anti-seize lubricant prevents future problems where steel bolts are screwed into aluminum parts. However, if you use a threadlocker on the bolt thread it will also help keep the bolt from seizing or galling. Just don't put both threadlocker and anti-seize on the same bolt.
How tight is tight? Some of the most used tools in my toolbox are the torque wrenches and the torque limiting screwdriver. My right arm isn't calibrated in newton meters, and yours probably isn't either. Too many parts have been damaged as a result of being made too tight.
If all else fails and you are faced with a bolt or screw that stubbornly resists all attempts at removal, there is a way to eliminate the bolt. Yes, eliminate. Some wel-equipped machine shops can do what's called electrostatic discharge machining. If you've ever seen the result of a lightning strike, you may have noticed that some materials have been vaporized by the lightning. That's what electrostatic discharge machining is like, only on a small, precise scale. Rather than using a cutting tool to remove metal, a small diameter wire is placed in proximity to the metal to be removed. High voltage is applied to the wire, and the resulting arc vaporizes the target metal. This process isn't cheap and some shops don't have the necessary apparatus. However, if you have an expensive part you'd rather not replace, it's an option worth considering.