FOUR Connect ring terminals have good quality shrink tube as isolator instead of regular rubber or plastic sleeves. Shrink tube protects the connection much better against oxidation than loose plastic or rubber sleeves. Many manufacturers make their terminals of brass that has more resistance than copper because brass it is much cheaper. With high current connections it would be wise to use real copper to ensure best connection and conductivity. All FOUR Connect terminals that are for 20 mmq or larger cables, are made of 100% copper. Connectors are rhodium plated to secure connection for years. This package includes 10pcs red connectors that fit to M8 bolt and take 20 mm2 cable.