Well it is fairly simple. I have a Bong Vodka bottle that I am going to turn into a bong. Usually I just wing it when making a bong but this bottle is a little expensive so I am being cautious. I have measured the down stem and it was ... 17/30" which is .56" Or rather it was that measurement with the grommet, the down stem it's self was 1/2" So the question becomes -> What size should I drill my hole? I have to buy a Diamond Bit to get this done obviously and I have researched the techniques to drill glass efficiently and I have plenty of practice bottle to play with. Everything is in mm when looking at the bits, .56" translates into 14.224mm. So should I get a 14mm bit? 13mm bit (which is .51")? I know everything is going to fit snug and the grommet with flex as you push the glass down stem in there and everything. So will 14mm be my answer it will still be smaller than the thing so everything should fit real snug yes?