I bought the Classic Bamboo Cap to replace the plastic cap on my Classic narrow mouth bottle, and it fits perfectly with the internal threads. I really like that the inside is 18/8 stainless steel, so my water never touches plastic, and the bamboo top looks great. It feels solid and seals well for cold drinks. Just remember it is handwash only and not for hot beverages, and it only works on Classic narrow mouth bottles. A simple, well-made upgrade.