Stem piece question

Hey, I want to make my own glass bong for the first time, and was wondering what the average diameter of a stem piece is for a bong as I want to drill the hole with a diamond drill bit to put my stem in.
Cheers ~ Derple

(By the way, if this is in the wrong category, feel free to relocate it)

There are a few diferent sizes for downstems. 10mm, 14.5mm, 18.8mm, and 29mm. The most common imo is 18.8 and also is the best.

Just get a grommet and make it big enough to hold a Diffy

NO Grommets! Rubber pieces are old school homes.



*but I guess in this situation since it's a homemade it would be apropriate

Well grommet to gong fitting of course. With a homage bong its fine.