select from the original 10mm stem as well as the newer 12mm stem. Cooling stem has 3 large cooling nipples where the vapor converges and is condensed and cooled down for your pleasure. The longer empty space at the front of the stem is intended for your stir stick to be inserted into the stem while not in use.
Be sure if you originally had a 10mm stem, that you stick with that unless if you have a 1/2" stem hole and can measure and confirm that. If you choose to upgrade to larger diameter, you will also need to purchase O-rings for 12mm. Please also be aware that some protector knobs may not fit well to a new stem if the vape is previous to the flared intake version.