Only when I have a problem. Besides when I first acquired the truck, I swapped them after I started having a misfire issue and found a couple to be fuel fouled. They were physically fine, mind you, I only replaced them because I wanted to give the ACDelco Iridiums a run to see how they would do. Conclusion is that they run the same as the factory double platinums, which was my theory from the start. A plug really should only be replaced when it is physically worn out, meaning that I can no longer be gapped to spec. Copper/nickel alloy plugs can be filed to restore the fine edge on their electrode and ground strap, and can be run until there isn't enough ground strap or electrode to form the correct gap. This isn't done anymore because copper/nickel alloy plugs are cheap, and it is far easier to replace them than to restore them. Iridium and platinum plugs can't be restored in this manner, and have to be changed when they wear beyond spec. This isn't an issue because iridium and platinum are much less susceptible to erosion than nickel, and so can maintain proper spec for many thousands of miles more than nickel plugs. The other advantage of platinum and iridium tipped plugs is that the tip achieves self cleaning temperature much faster than nickel plugs. This is due to the much smaller diameter of the platinum or iridium tip, and makes the plugs less susceptible to fouling. This same behavior makes them less desirable in high performance applications where proper heat range is far more critical than service life, and where the platinum or iridium tip can become a potential source of preignition events.
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