I'm looking into getting a pre-cooler of some kind, but I'm uncertain of what size my downstem is. A penny fit perfectly into the opening, if that's any help. I'm just not sure how to determine the size of it.
there are 2 mainstream sizes of downstem diameter for pipes; 14MM and 18mm, less common is 9mm. Most precoolers are attached to GoG systems, do you have a Glass on glass pipe? Also pics would help or a description of how the downstem is set up.