Bong slide hole diameter

[ThinkLong]

So I just made a bong in pottery and would like to drill the hole in it for a slide, but I need to know what size hole to drill.
Is there a standard slide diameter? Or should I just buy a slide first and figure it out from there?

[brad1028]

as far as i know there are two common sizes 14.5 or 18.8 but if you are getting the down tube from a headshop just ask them first what it is