Put plugs in my 273 yesterday. I had 8 plugs from a junkyard Magnum along with wires, heat shields, and coil. 6 were Bosch Platinum+2 4308 (4 ground fingers) and looked new. One was the much shorter 4302 (fix for oil fouling?). The last was a Splitfire SF522D.
The SF was eroded to a 0.080" gap, so I tossed. I think bogus anyway. Ever read their ads? "First we dyno'ed with regular plugs" (cold engine?) then we dyno'ed with SF plugs and got more HP (engine hot now?).
I added two Autolite 66 plugs to the Bosch 4308's, exact same threaded length, though use the small socket. One can't gap the Bosch's, which is equivalent to 0.040", based on the gappable plugs listed on rockauto. Seems weak since I thought good electronic ignitions typically like a 0.060" gap. Maybe only if distributor-less. Magnums lasted until 2003, still using a distributor. I have a Crane XR700 ignition and will later add an MSD Stacker, so set the Autolites to 0.055". I recall the old Delta CD box on my Newport said to gap to 0.050".
The Magnum plug shields don't fit my 273 head. They are simple metal sleeves with a gap that press into the plug recess. They might be supported on just the wire boots, but might rattle. Anyway, the wires from the 273 were fairly new silicone ones so I re-used. I might use the Magnum coil since it looks like an after-market E-core type and might put out more voltage. It was bolted to the front of the left head, I assume for better cooling, though I don't have the tapped holes in my head.
