Just a quick reminder here that when the iridium plugs came out they had a much bigger gap setting and ford were having problems with both early plug failure and coil packs failing especially in the coil pack rather than the coil on plug set up. They struggled for quite some time to resolve the issue but eventually discovered it was due to the plug gapping being to large. There was a point in the early days when plugs were being routinely changed out annually regardless of milage even though they were rated for 100,000 miles or more. Ford then stated a gap of 1.0-1.2 mmm which resolved the issue. The dealer instuction was to set at 1.0mm.
I changed my Iridium plugs at around 40k, but this was because they'd been in since new and were 7 or 8 years old, the OE FoMoCo that came out were clean as a whistle and the gap erosion was only a tho or two, pretty amazing when you think of the hundreds of millions of times they've fired
