All-mountain SLX HollowTech II chainset is tough and aggressive yet also lightweight and excellent value The HollowTech II Chainset features hollow forged crank arms which are both lightweight and rigid whilst being incredibly strong and durable HollowTech II integrated bottom bracket design uses a wide bearing placement with a large axle diameter for increased stiffness, efficiency and added durability 70 degree spider arm for optimal rigidity 36/26T combination 12-speed chainrings We recommend that the bottom bracket is initially installed by a bike shop to ensure perfect facing and fit For use with 12-speed HG MTB chains only Boost 51.8mm double ring chain line for use in conjunction with 142 mm O.L.D.