TMS Kobo Original Acoustic Improvement Goods Rotary Spacer-kun (Brass Torched) These are acoustic improvement goods to be inserted between the rotary caps of a horn. They change the ease of sound production and resonance. They can be used with Yamaha, Alexander, and Holton horns. I think they can also be used with tubas and trombones. Outer diameter: 21.5mm, Thickness: 1.5mm, Hole diameter: 10.5mm Weight: 2g Made of brass