These spring kits are designed to give proper pressure when used with cams in the .550″ – .675” lift range. The springs are manufactured from Chrome Silicon wire for maximum durability and longevity in street engines. Full sets include chrome moly or titanium top collars, steel lower retainers and chrome moly keys. Some rocker box clearancing is required. The lower collars supplied with these kits can be machined to accept Zippers “Step-Lock” guides, or you can purchase collars listed separately. 1.530″ O.D.