RK's traditional round O-ring chain is a convenient blend of performance and affordability. SO's O-ring is constructed from Nitrite Butadiene to seal in lubrication and deliver smooth power with minimal seal ring drag or loss of horsepower. 520SO chains are suitable for low to mid-range CC motorcycles that came from the factory with sealed chains. 520SO can be used for both on street and off-road up to 400ccs. 520SO is available only in natural finish.