All the others looked practically new (other than the same light surface rust & dirt/oil on the exposed ceramic), much like this one would if its electrodes were the normal shape. The truck only developed a noticeable miss yesterday, so today we changed the plugs & wires, and cleaned the cap & rotor. Now it runs just like it did a few weeks ago - perfectly!

But I can't figure out what caused this... If you have any ideas:
http://classicbroncos.com/forums/showthread.php?t=152008

Owner caption December 2009 · Photo 427 of 689