Hey man, I'm still stuck. After searching everywhere, it seems like Bykski does use the imperial system and metric system. How do I measure the fitting? Do I take in consideration of the o-ring? Or just the whole OD? I can find Bykski fittings that are 12mm, 1/2 inch, and 14mm in OD. 14mm and 1/2 seems to be the most popular. I'm mainly just stuck on how to measure the fittings, because if you take in consideration of the o-ring, it's around 12-12.5mm. If you measure the whole OD, it's around 14mm. Thanks for your help, it means a lot.