skanking biker--

don't know what you're unclear on but here's a quick primer. threadless stems for the kind of bikes we talk about here are generally one of two sizes-- 1 1/8", or 1". that refers to the diameter of the steerer tube, which is the part of the fork that goes through the headtube. there are other important measurements--the clamp diameter, which is generally 26mm and refers to how fat the bars should be where the stem grips them [stems w/ removable faceplates can hold bars a bit bigger or smaller, though]. there's length of the stem. that's straightforward. and there's also angle.

a google search for bianchi pista brought me here:
http://www.bianchiusa.com/05_pista.html

...which told me that it has a 1" threadless headset. which means that you need a stem for a 1" steerer tube.

hope that helps.