6pcs of well nuts are made of brass and rubber sleeve, great accessory for your kayak, canoe, boat. Features: 6pcs brass nut with rubber sleeve. By tightening a threaded screw in the brass nut, the insert expands to secure the fixing. Great accessory for kayak, canoe, boat. Specifications: Material: Rubber + Brass Size: M4 / M5 / M6 (optional) M4 Dimension: 13 * 11mm / 0.5 * 0.4in M5 Dimension: 17 * 14mm / 0.7 * 0.5in M6 Dimension: 21 * 16mm / 0.8 * 0.6 in Package list: 6 * well nut (screw is not included)