Okay, now I'm confused! (i'm trying to find out the same measurement on my bike.) now, do i need to measure the outside diameter of the steerer tube, the inside diameter of the of the headset cups, or the inside diameter of the head tube without headset cups? also, when i take off the nuts and washers and sundry off the top of the steerer tube(i'm using a threaded headset right now) should there be a lot of play in the fork?

[edit] after looking at it more closely, i get the distinct impression that the headset cups are extra thick or something so that a 1" threaded steerer tube can be used with a 1 1/8" headset, but that could be my inexperience talking.