I have been searching for this exact size o-ring for some time now to replace the o-ring on the Wotofo Serpent RDTA that holds the top cap in place (the o-ring that sits flush with the deck, right above the filling ports). This o-ring wears out quickly so it's good to have a replacement on hand. Every other o-ring I have tried on this site is either too big or too small, but this one is the precise one needed.