I wanted to get a glass bong to serve as a bubbler on my vape. I also wanted to use this bong for concentrates so I was looking for a nail that would fit the same bong. I imagined my rig (for vaping) would be set up like this: Vape hooked up with DDave mod, soft rubber hose going from DDave mod to ground glass 18mm joint, 18mm joint hooked up to downstem of the bubbler (bong). After a lot of research I placed my order with a reputable OLHS (can't stress the reputable part enough - check the store's reviews). I ordered an 18mm nail and a bong that had an 18mm opening. Here's the funny part: the port that took the downstem was 18mm but the joint in the downstem was a 14mm. Now for the happy ending: The nail they sent me fit in the downstem joint - did they see my error and correct it? I'll never know. The main reason I'd bought the rig though was for my vape and there was no way my 18mm elbow was going to fit in the 14mm downstem. However my soft rubber tube fit snuggly inside the downstem. Generally I'll wet the terminal end of the tube coming from the DDave mod prior to inserting it in the down stem. This gives me an airtight seal and despite my incompetence in ordering my rig set up is exactly the way I had wanted it. For bonus points the tube going directly into the downstem is much larger in diameter than the terminal end of the elbow joint so it flows much more freely than my original intended design.