Just to inform anyone who picks this up, Ford decided some time ago that a 1mm gap was the optimum for their engines when using iridium plugs and sent out an urgent internal instruction to their dealers. originally it was at 1.3 mm and was destroying plugs and caused problem running. After several test periods they found that 1mm was the optimum gap for consistency of plug performance and engine performance. For the old school that's 39 thou". I can't remember what they advised as a change time milage but it wasn't anything like the service an iridium plug is capable of and people do get 100,000 miles +with little wear or degradation to the plug. perhaps the problem lay in the fake iridium plugs that flooded the market and still do. If they seem like a bargain then its odd's on that they are fake so only buy from an accredited distributer. Fake's are lucky to give you much in the way of milage and your first symptom will be difficult starting. If you pull a plug you'll find the fake iridium plug will have eroded it's center contact to nothing as its probably made of copper!
