"PP" Indicates it is a double platinum plug, i.e., platinum
on both the center and outer electrode. Ford recommends
these for replacement in engines with waste spark ignition.
The plugs installed at the factory are single platinum
plugs, but there are two different types. One with platinum
on the center electrode and one with platinum on the outer
electrode. You only really need platinum on one electrode,
but it has to be the proper electrode for the polarity of
the spark. For waste spark ignitions half the plugs have the
spark jump from the center electrode to the outer electrode,
and the other half have it jump from the outer electrode to
the center. In a factory situation, they can install the
proper single platinum plug in the proper location. For
after market, it is just easier to sell double platinum
plugs. That way you don't have to worry about mixing them
up, and you don't have to stock two different plugs for each
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