I have this bong here: http://www.brotherswithglass.com/HVY-Glass-10-In-Straight-Water-Pipe-p/bwg-hms1.htm not sure which downstem size to get because it says 14/18mm which is confusing me... i just broke all the diffuser slits off of it so it still works perfectly fine but without diffusion.
Looks like you can use a 14/18 or an 18/18 downstem, but the important part is it looks like it's only 3 inches long....