Set of 3; Polished Ebony With Felt, Inner Diameter: 4.25″ Outer Diameter: 5.5″ Benchworld’s. Premium X-Large Solid Wooden Caster Cups in polished ebony finish are the best choice for a Grand Piano, with a new design that has three ripples on the side and a wider inner diameter for bigger wheels. The outer diameter is about 5.5″ and the inside diameter is 4.25″. 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.