I have 8 in use elsewhere in my test loop already (actually the two I tried to put on my D-Tek, since I still have two in the plastic). (so I doubt any are 5/8, or else I'm boned, haha0

All at or just under 20mm.

Measured a spare BP 3/8(1/2) I have here and it's definitely larger at the base (21mm)....sigh....I have no clue why mine weren't working. When I disassemble tomorrow I'll investigate further, maybe I was being too hasty or maybe the natural curvature of the tubing was forcing them toward each other.

I'd really like to be able to keep my fittings consistent through my testing and getting the Koolance fittings to work on the D-Tek would be sweet...esp since they don't have the same ID