The cap fits great and I haven't had any problems with it
I ordered this to go with my 21oz standard mouth Hydro Flask bottle. The cap fits great and I haven’t had any problems with it. No issues with leaking or any “whistle noise” while drinking. The flip straw makes it much easier to drink than having to unscrew the original cap that came with the bottle. I have also found no loss of the bottle’s capabilities with this cap on. I have left water and ice in the bottle over night and found that the water was still ice cold after 14 hours, and there was still ice in the bottle. I really like this product.
Review by on January 27, 2016