Denso Iridium Tough Spark Plug VKH20. Manufacturer Part Number 5618. Denso Iridium Tough Spark Plugs use the same 0.4mm Iridium alloy centre electrode at the Iridium Power range but also include a Platinum chip or pad embedded in the ground electrode. This extends service life considerably whilst maintaining optimum ignition efficiency. Materials used in the construction of the Iridium Tough range are of extremely high quality which means reactions to chemicals in fuels such as Liquified Petroleum Gas (LPG) and wear in general is much less likely than with lesser plugs. These plugs are ideal where service intervals are extremely long or engine bay access to change the spark plugs is difficult. Useful service life for most applications will be 60,000 miles or longer. Made in Japan.