What is the diameter of a 2 liter bottle cap?

This will differ depending on the type of bottle. Even though the capacity of the bottles may be the same (2 L), the design of the bottle and therefore the cap used will be different.

The diameter of the cap on a 2 L Coca-Cola bottle is 28mm.

To measure the diameter of the cap that you are using, place your ruler across the middle of the cap and measure the length from one end of the cap, through the middle to the other.