I just bought 2002 Specialized Expedition Elite 3×9 speed bike. The bike comes with stock threaded stem. It was taken apart for service. And upon reassembly the bottom of the stem is catching and not fitting inside. Nothing has changed in the setup, all parts are the same, no bearings fell out, nothing broke, so I am at loss. I will be grateful for any help!
If this is a quill stem—with both a horizontal member and vertical member that fits in the steerer tube—the part that fits in the steerer tube will have either a wedge or expanding bolt at the bottom, which is how it locks inside the tube. You need to loosen this at the top of the stem and make sure the wedge/bolt is loose at the bottom.
I will note that occasionally, when servicing a quill stem, a newbie will completely unscrew the fixing bolt, leaving the wedge (that the bolt sticks into) in the fork stem.
If the wedge stays put then it will all go back together OK, but often the wedge will come loose and fall farther into the fork stem, and maybe get twisted sideways or some such.
If this happens, either turn the bike over and shake out the wedge, or remove the bolt from the quill assembly and thread it into the wedge to "capture" the wedge and allow its removal.