The 40oz Wide comes with a Stainless Steel Loop Cap that is 100% stainless steel on the inside, so your drinks never touch plastic. The Mouth opening is 2" in diameter. All Klean Kanteen products are made from high quality, 18/8, food-grade stainless steel that's totally free of BPA and other toxins. Because stainless steel itself is safe, they don't need to coat the inside with a plastic or epoxy lining, which is one of the main differences from aluminum bottles. No matter how many times you refill it, no matter what you put into it, your Klean Kanteen will keep your drinks fresh, crisp and clean. It will also nest in the Glacier GSI Mug, Olicamp Mug, and the Vargo Mug!