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Thanks Leif!

I’m not too worried about overheating as I would have thought a water-cooled head would keep things reasonably in control.  An aircooled engine may well be different though!  The D21 plugs have a very long tapered electrode with a thin and pointed porcelain insulator so they “look” like a very soft plug. The AC87 “look” much more conventional and harder.  I don’t know if that means anything though 🙂