Part 2: other tools
In measuring outside diameters of standard size shafts on our bikes, we find it useful to accurately measure diameters 1" and smaller. A cheap caliper is NOT up to the task, and high quality caliper is, but a micrometer is best.
For most things, a .001" mic is good. You can find something used if it is in good shape, but it is generally cheap and time-efficient enough to order a new Fowler or SPI mic for $20-30, which will read in thousands and have subdivisions so you could estimate the tenths of thousandths.
The above mic is a Mitutoyo digital micrometer. the first three digits are thousandths, the four digit is tenths of a thousandth, and the fifth digit is half-tenths (50 millionths of an inch). You pretty much don't need that fifth digit for motorized bicycle related stuff, but it's good to know what it is. Non-digital mics use a vernier scale for reading tenths. You can look up on the internet on how to read micrometers. Generally very easy!
Always remember to have a super light feel with mics to get the best repeatable reading. I use the bottom two mics the most, the green one I will take with me to Home Depot if I need it, if I lose it I won't cry about it cause it's damaged.
The greatest use from micrometers is when you take a screw out of something, measure the OD and write it down or remember it, and remember if it is a fine or coarse thread in case you lose the screw. Let's say it measures .113" and has a fine thread, it is probably an M3x.5 screw. When looking for a longer or shorter screw, measure the diameter and match them up within .0015" up and down, then hold the screws up to the light to see if the threads mesh. Easy way to see if your screw will match to what you need.
Another thing you should invest in is a drop indicator. This has a dial similar to a dial caliper, usually a tenth of an inch (100 thousands per revolution) and they generally have a range of just over one inch: 1.05" or 1.1".
This particular one I picked up from an older manual machinist who had two of the same, he sold me this one for $25. It is an older Starrett though scratched up/dented condition. I made a mount for it to true my rear wheel. Very easy, just slap it on there is true your wheel, don't even need to take the tire off like on a regular truing stand. Get it within .010" or .005" if you're a perfectionist and enjoy running perfect wheels. Same goes for sprockets, clutches, disc brake rotors, whatever!! Find one for a good price and jump on it.
I hope this guide will help you make more accurate assemblies and parts, any questions just ask. Between a 6" caliper, 1" mic, and 1" drop indicator, you will be covered for almost everything requiring accurate measurements. My buddy sure got more than what he bargained for!!