Standard size .5620" All Kibblewhite 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