When manufactured, both the Zetec and the Duratec were assembled with single-plat plugs on one side. The double plats were needed on the other side of the coil's polarity used doubles (or fine-wires) due to the opposite polarity of the spark in the waste fire system. Using the single plats where they were sufficient to operate correctly saved a little money on every car that Ford built. Over the entire production run, the cost savings were significant. Ford used this same strategy for most of their engines with waste-fire systems.
Replacements are done with either double plats or finewires, eliminating the need to stock and to install two different plugs when changing them. At the retail level, the cost per job is not significant.
I asked for Motorcrafts last week at my local AZ store for the wife's car. They didn't have enough on hand so I ended up with APP764s which were on hand. The old ones were still running great at about 50K miles but I have emissions tests this month that required a new O2 sensor. Since I was already under the coil pack, it was simple to change the rear bank at the same time. Wife's new job is now an 80 mile daily round trip, so, hopefully, the new plugs and O2 sensor will keep fuel economy on the upper end.
I even put sythetic oil in it so I don't have to change as frequently.
Steve
