These valve guides are machined with a superior surface finish from C630 bronze, which is composed of 82% copper, 5% nickel, and 10% aluminum. This material is harder than the OEM, due to the nickel content, and offers excellent wear resistance. The addition of aluminum increases the heat transfer characteristics of these guides, which allows for longer life. In combination, these elements make KPMI® guides stronger and tougher than OEM. The bores of these guides are intentionally left undersized, allowing each builder to ream and hone the ID for their desired clearance after installation in the head.