Vernier calipers are useful for measuring inside and outside diameters. They also work fairly well for figuring out chainline dimensions, measuring spindle lengths, and sizing loose bearings. You can get them for less than ten bucks at a building supply store. I prefer plastic versus metal because they're easier to read and they don't scratch the material. Just make sure it has metric measurements. The real trick is learning how to use it:

http://hyperphysics.phy-astr.gsu.edu...b/vernier.html

or

http://regentsprep.org/Regents/math/tools/vernier.htm

Probably way more information than you wanted...