Cast Iron Intake Valve Guide Kibblewhite All KPMI Guide profiles are designed to have a minimal effect on the airstream while maintaining critical cross sectional thickness in high-temperature areas, which promotes high-performance and endurance. Strict attention paid to ID/OD and cylindricity call outs. C630 Premium High-Performance Bronze: Copper based C63000 nickel-aluminum bronze material used to make premium high-performance bronze valve guides because of its ideal characteristics for the application. Cast Iron: Kibblewhite uses their own specification of cast iron made exclusively for them. They specify elements such as carbon, silicon, manganese, nickel, chromium, copper and sulfur to produce a highly pearlitic structure with dense fine grains for excellent wear resistance.