How many times have you needed a particular bolt and found that the only ones you had were either too long or too short for the intended purpose? We'll it happened to me (again) about 15 minutes ago and prompted this particular tip. What usually happens is that I'll run into a situation where I need an odd size bolt. Most of the time it's because I'm doing some sort of silly modification or installing an accessory and need to replace a piece of OEM hardware with something else and the something else is only available in an in between size.
Lengthening a bolt is fairly difficult, and not something I'd recommend doing. Lest you think I'm pulling your leg, I'm not, a couple of times I've had to extend a bolt in an emergency situation, but the finished product was something I wouldn't trust as far as I could throw it. However shortening a bolt is fairly easy.
Measure the bolt.If you're replacing an OEM fastener make it the same length, if you're just shortening one for appearance sake, as you might if you were attaching an accessory, calculate the length you'll need including all the washers and a nut and making certain that at least three threads stand proud of the nut after everything is tight.
Run two nuts onto the bolt, making sure they're well below the portion you plan to cut off. Run the first nut up your cutting point, and use the second one to lock in place.
With the nut as a guide, use a hack saw to carefully cut the bolt to length. If you've got a bench grinder or sander handy, dress the cut edge until it's perfectly square, even if it means grinding away a bit of the nut. If power tools aren't handy use a flat file or some emery paper.
To reshape any dinged threads screw the first nut off the bolt, then screw the second one almost but not quite all the way off, before running it back down the bolt, repeating the process until the nut spins smoothly off and on.
Because there's no lead left on the bolt it may be a little tricky to start on the first attempt, but work carefully and it should thread in nicely, if it doesn't stop and recheck your work, you may have a damaged thread, in which case you'll either need to run a thread chaser or die over your cut bolt or try again.
Let me leave you with a word of warning here. This is a fine trick to use when the bolts and what they're holding aren't particularly critical, but because a bolts tightening ability is in part predicated by the ratio of its threaded to unthreaded length a cut bolt is unpredictable as far as strength goes, so under no circumstances should one be used in a critical situation.