A watch case tube is the insert located on the side of the watch case where the stem goes through and the crown sits on. A watch crown with an attached stem goes through the case tube to attach to the watch movement where it controls many of the settings of the watch. The case tube is an important watch movement part that needs to be sturdy and strong because it is a watch part that protrudes from the watch case and is likely to be easily bumped or hit. Attach the case tube by threading it into the watch case or friction press into the case. The case tube will have to match the crown to assure a good fit so that water and dust do not easily enter.