The Ultra Pro Magnumâ„¢ XD Rocker Arms are engineered from durable 8650 Steel that outlasts and outperforms other inferior knock-off and import Rockers. A unique machined billet Pushrod seat insert offers exceptional durability. These new Stud mount Rockers are fully rebuildable and use precision-sorted needle bearings and hardened Roller tips to better distribute the load and reduce wear for a longer service life. In addition, they work with most diameter springs and retainers. The XD design utilizes advanced FEA and CAD design and development to improve strength, stiffness and MOI optimization for endurance, drag and circle track Applications.