Hydro Flask Wide Mouth Flat Cap Hydro Flasks Wide Mouth Flat Cap for Hydro Flasks Wide Mouth water bottles. Replacement cap compatible with both 18oz, 40oz and 64oz Wide Mouth bottles. Made frompolypropylene (PP#5) which means they are BPA-Free The Lifetime Warranty makes Hydro Flasks the best water bottles Please note that the Flat Cap does not fit on the older style Wide Mouth flasks. Only compatible with Hydro Flask Bottles.