On a keg (50L), clamping it to the top of the keg with the spear removed
This would be for inverting the keg, using the neck of the keg as a bottom drain, then cutting a hole in what used to be the bottom (now the top) and adding a corny lid and bulkhead ball lock posts to use as a pressure fermenter
2" (64mm OD, whatever you want to call it) is what the majority of still owners use if they're just clamping a column to a keg, most people who build their own gear either use 2" or 4" for their columns and condensers (if they're not making liebig condensers)
I know I could use 3/4" threaded, but as 1/2" is the common thread size for home brewing, I'd like to stick with that so I have everything the same size
It's only recently that I've seen 1.5" gear coming into play, I've wondered where that comes from