Perfume Studio Metal Balls for Roll on Bottles Bulk Set of 48 Units Replaces Roller Tops with Stainless Steel Metal Balls. Fits Roll on Bottles with a .63 in / 16 Mm Inner Neck Diameter Set of 48 Units of stainless steel ball applicators roller tops. Will only work on 16mm / .63 inches inner neck bottles Please make sure it will fit your bottles before you buy! Unlike plastic balls, metal balls ensures a smooth application on your skin. NOTE: Use ONLY with pure uncut oils (Such as Perfume Studio Pure Oils) . Not recommended to use with carrier oils containing lubricant oils such as coconut, almond, jojoba, etc