RK 520MXU is the lightest Sealed-Ring enduro/motocross chain available in market today, 13.8g/link, only 0.1g/link heavier than RK 520MXZ4.

MXU series is the next generation of enduro/motocross racing chains featuring RK’s only technology of Ultra Thin Seal, U-Rings or UW-Rings, make possible to design a Sealed Chain with
the weight of a Non-Sealed Chain. Seal Rings prevent debris to enter the chain
which ensures the efficient horsepower transmission from the engine to the
rear wheel throughout the chain life.