Buna-N (synthetic rubber) stop material in sizes to fit all brass instruments. Not affected by oil, grease, or solvents. This material is much softer than vacuum cleaner belts or o-ring stock. When properly installed it will not bounce, compress, or tear. Much more stable and positive than cork stops. Use .210" for Paxmans, Holtons, Schmids, and most other horns and trombones. Offered in standard 50 duro and extra-soft 40 duro. Our .188 stock fits Conns, usually with no trimming. Available in 70 duro only. For a softer feel use .210 in 50 or 40 duro. .250" is needed for some Alexander change valves. .250"-.313" fits most tubas. Trim to fit with a new single edge razor blade. Sold in six inch lengths.