Wrenches

This article isn't really bike specific, so feel free to skip it if you know how to use a wrench. If you're new to taking things apart then read on.

There are a number of tools you can use to remove bike parts. I'll try to demonstrate the common ones you would use for basic maintenance and then some. Let's start with wrenches. Older bikes have lots of hexagonal nuts and bolts (generally called fasteners) to hold things together. To remove them you will want to use an age old tool called a wrench:

An adjustable wrench is better than the wrong size wrench (like using a 5/8" wrench on a 15mm axle nut), but it is less than ideal if the correct wrench is available. You can use an adjustable if you're on the side of the road and need to fix something, since it's easier to bring a 6" adjustable with you than a whole toolkit. However if you're at home and you're working on a valuable bike, you shouldn't risk damaging the fasteners with an adjustable.

It's best to use the closed end of a wrench when breaking a bolt loose, since this will distribute the force onto all 6 sides of the fastener and prevent it from rounding off. Alternatively you can also use sockets:

I normally prefer wrenches over sockets for bike work, but a ratchet and socket is one of the best tools to remove a crank bolt since a wrench won't fit inside the crank. As far as tool quality is concerned you definitely don't need anything expensive like Snap On or Matco. Basic chrome vanadium wrenches from the hardware store will do fine as long as you're using the right size.

If you don't already know what size a fastener is, use the tightest fitting wrench you can find. Most bike fasteners are metric (except for older American-made bikes, these usually have 1/2" or 9/16" stem, handlebar, and seatpost clamp bolts, and occasionally 1/2" front axle nuts. The rest of the fasteners will probably still be metric). If a wrench feels a bit loose, try the next smaller size. Things get a bit confusing when you're unsure whether the nut is English or metric, but if you're willing to do some math you can always find the right one.

Math Warning 

To convert from English to metric, just divide the fraction, then multiply by 25.4 (there are 25.4 millimeters in an inch). For a 9/16th" wrench, divide 9 by 16 (this gives you .5625"), then multiply that by 25.4 (this becomes 14.29mm). As you can see, a 9/16" wrench is just a bit larger than 14 millimeters. If both of these wrench sizes easily fit on a bolt, use the 14mm because it is a tighter fit.

End Math Warning

In general, bike fasteners are usually soft metal, and get pretty chewed up if you use the wrong sized wrench. Also, avoid using pliers or anything with serrated jaws on nuts or bolts:

These are meant for different purposes, and are a sure way to ruin a fastener.

Finally, most modern bike parts use Allen bolts instead of hexagonal ones. These have a small hexagon stamped inside the head of the bolt, and they require Allen keys:

These are pretty straightforward to use, just make sure that the key is fully seated in the bolt before turning it, because these can round off easily (especially when they get rusty). You will never see an inch-sized Allen bolt on a bike, so get a metric set instead.

As far as bike wrench sizes go, the most common wrench sizes are 8, 9, and 10mm for brakes, derailleurs, and other smaller parts. 14 and 15mm are common for axle nuts, seat clamps, and pedals. Finally, 4, 5, and 6mm Allen bolts are also common on many modern bike parts.