Denso Iridium IK24 Spark PlugDenso Iridium IK24 Spark Plug Specifications Whatever your use or engine set up the correct choice of spark plug is very important. Iridium is very heavy and dense metal ensuring that Denso spark plugs have a longer lifespan than OEM ones withstanding more than 100. 000 km in standard use. Which heat range to choose ? If the letters (for example IK) indicate the spark plug's shape and thread size the numbers (from 20 to 27) indicate the heat range