Natural Steel Finish Clip Type Connecting Link for RK 520SO O-Ring Chains. O-ring seals are made from an advanced Nitrile Butadiene composite for maximum performance and wear life. Nitrile Butadiene O-rings seal in lubrication and deliver smooth power with no O-ring drag or loss of horsepower. The 520SO is one of the best O-ring chains in the industry. It's always smart to have a spare master link in your garage or repair kit.