NGK Spark Plug Wires. Built to meet stringent OEM tolerances. At the centre of NGK Resistor Spark Plug Wire Sets is a fiberglass stranded core, made from the same family of materials used in bullet proof vests. This core greatly enhances the overall strength of the wire, providing a super strength backbone on which the various layers of the wire assembly are formed. The core gives protection from damage caused by the pulling strains that wires are often subjected to and that can destroy their performance. The cords come in blue and are number to aid installation. Perfect for D-series engines.