Brad C.wrote a review of Hydro Flask Standard Mouth Sports Cap on August 2, 2013
Familiarity: I've used it several times
This top fits with any of the Hydro Flask "standard mouth" bottles. This is the one that I use it with: http://www.backcountry.com/hydro-flask-21oz.-standard-mouth-water-bottle?ti=U2VhcmNoIFJlc3VsdHM6aHlkcm9mbGFzazowOjM
IF you have a full bottle of water with this cap, be sure that you store your bottle upright otherwise water will leak out! The leaking comes from the little gasket/rubber piece that is located in the loop or circular portion that you can see in the picture. Without this air hole you would not have any suction so thats why its there! Other than the leakage, it is a great piece to have especially if you're on the go.