I just replaced the plugs on my 1996 4.0 Ranger. They were OEM Motorcraft plugs - likely the first replacement plugs in the engine at
322000km plus. (the dervice records show plug replacement at just over
120,000) I found it interesting that 3 of the six plugs did not have the ground electrodes installed centered on the center electrode - and one of those 3 was on the "positive firing" side of the coil. It had the one side of the ground electrode burned off right round. The other
2 positive firing plugs had a nice round depression eroded out of the center of the ground electrode. These were single platinum plugs. On my last Fords (1990 Aerostar 3.0), 1989 Aerostar 3.0,and 1996 Mystique
2.5) they always had double platinum plugs - hellish expensive little beggars!!!. The Ford dealer wasn't open this afternoon,so I put in NGK G-Power plugs this time. Figured with the QC on the last set of Motorcrafts, the NGKs were likely just as good a bet (and all six WERE well centered when I checked).
See if the intermittent missfire at idle is gone now - and mabee even a bit better gas mileage???