You should look into getting a set of cheap plastic vernier calipers. They are handy for measuring all kinds of round bike stuff (inside and out) if you are wondering what size component will fit int your setup. They measure to a tenth of a mm easily and accurately, which is handy because lots of bike dimentions like 27.2 mm seat posts go to tenths. Your hardware store will prolly have some for $2 or so. The set in the pic there is nicer though.