BOSCH does NOT make plugs in the correct heat range for the Duratec (probably Zetec either)
BOSCH's design for thier ground electrode is garbage. I have seen far too many center electrodes melted or just dropped out!
I ran +4's on my car for 6k miles (they were free) The reason they were in so short of a time is the entire back bank failed badly! The front bank had bad wear too. (the engine had cut out problems!)
I wisened up and decided even for free I should have stuck with my experience and skipped any BOSCH plugs.
I installed Motorcraft plugs (AWSF22FS - 1 step colder - that's another story) and have been running smoothely ever since...
Also - DO NOT buy those cheap ramp style plug gappers. The are not very accurate & become even less so over time. Also when you force them between the 2 platinum electrodes you can damage and prematurely wear the platinum tips.
Buy a normal feeler gauge & gap tool for the extra dollar or two...
