From the 2005 H-D Catalog: Double Platinum spark surfaces offer superior erosion resistance even after millions of sparks. H-D Double Platinum plugs also feature a patented semiconductor suppressor, a triple-coated shell with special anti-seize coating, copper core center electrode for high thermal capacity and peak performance at all engine speeds, tapered center and platinum side electrodes for maximum combustibility. Special high alumina insulator provides self adjusting temperature for maximum resistance to fouling. Harley-Davidson's longest lasting spark plugs.