Broken bong downstems are not the end of the world for your bong. First, you need to assess the damage. Where’s the damage located and how badly damaged is your downstem? Once you have finished your assessment, you can decide on how to tackle the repairs.
Duct tape can be used if your downstem has clean breaks, and/or if there are only less than three pieces to join together.
Note that this is only a temporary solution, especially if the broken piece is submerged in water. Duct tape will thicken up your beaker bong downstem. You may have to remove the grommet at the joint for the downstem to fit through. However, do not expect the same seamless, airless junction as before.
Bonding agents such as food grade silicone or food grade epoxy give a semi-permanent solution to your broken downstem.
There are other industrial strength bonding agents that you can use. However, use them carefully as they contain toxic substances that can be dangerous to your health.
The best and almost-100%-permanent way to fix your downstem is to use a blowtorch. Torching also gives you the option of attaching small broken pieces in the bong downstem. However, this is best left to professionals because of the extreme temperature and fire hazard. Should you wish to try this, make sure that you did all the necessary precautions against fire accidents.
Use a torch with caution. A torch can be used for removable bowl stems. There’s a risk that the high heat will crack your bong’s water chamber.
Study your chosen method well when fixing your own downstem. You can hire a professional for a cleaner fix, but that will cost you a hefty sum. Depending on the damage, you may be better off buying a new bowl downstem.