Kibblewhite Precision machining valve guides are made from the highest quality materials available. Each individual part is inspected during each machining operation. Outside diameter is precision machined between centers to ensure concentricity. Each part is clearly identified to facilitate easy storage. For each type of engine there is an extra large “Repair Type” oversize guide available. Inside bore diameter is .001″ smaller than the valve stem diameter and must be reamed to size for a perfect fit and long life. C630 is a blend of copper, nickel, and aluminum. The high copper content gives excellent thermal qualities, the nickel makes the guide tough and wear resistant, and the aluminum brings work hardening characteristics. This bronze alloy allows for tighter clearances than other materials, creating better valve and seat sealing, as well as superior oil control. The expansion qualities of C630 bronze make it especially compatible with aluminum cylinder heads.These superb guides are the best money can buy. Sold each.