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If anyone is struggling to find a colder plug for their Duratec than the heat range 5 and 6 listed in the parts books, then I have some good news for you. The NGK TR7IX and TR8IX are iridium resistor plugs, with tapered seats, projected tips, 5/8" hex, and are heat range 7 and 8 respectively. Although I couldn't find them in stock locally, they are available from www.sparkplugs.com for $7.60/ea. I installed mine this weekend (TR7IX) and so far, so good.
-John
-John