Set of 3; Unfinished With Felt, Inner Diameter: 3.5″ Outer Diameter: 5.5″. Benchworld’s X-Large Solid Wooden Unfinished Caster Cups are the best choice for a Grand Piano. The outer diameter is about 5.5″ and the inside diameter is 3.5″. This item comes in a set of 3 with black adhesive felt. The felt will be easy to slide whereby the rubber makes it harder to slide the piano.