Ronstan Low Friction Ring - 21mm Hole. They are best suited for systems requiring relatively small line deflection angles or for high static loads. The design achieves a nicely rounded internal profile for smooth control line trim under load and a deep external groove to retain the rope securely within the large shoulders when spliced into a loop. Specifications for Ronstan Low Friction Ring: Manufacturer: Ronstan Color: Black Package Contents: Ronstan Low Friction Ring Ronstan RF8090-21: Low Friction Ring - 21mm Hole