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Mike, Myself n others have spoken on this subject for years, not sure if everyone ever agrees lol its almost like politics. Id agree the 3116 is better then the 386 in an antique tractor application cuz its a NON RESISTOR plug and far as I know its heat range is somewhat higher then a 386 sooooo o its BOTH hotter and non resistive. As far as the gap, I see that as very much related to the rest of the ignition system, especially the coil, as a primary concern regarding whichever plug one chooses. As youre probably aware as the gap widens the spark can change from a hotter blue to a yellow spark, so the best trade off where the spark is plenty hot but the coil voltage is operating where it and the rest of the system was designed for probably yields the best gap choice. Only way I see to teat all this would be hooked to a Dyno but thats also gotta be weighed against the coils specs and its service life as if its overworked she may develop a few more HP but at the expense of earlier coil or points failure????? ??? Sooooo ooooo o all that being said, and as the owners manual recommendation is based on the entire ignition system (especially the coil) and its capabilities n limitations..... .. and the gap distance is more important then whatever brand plug is used,,,,, ,,,,, ,, I AGREE WITH RUSTY, ID GO WITH WHAT IT SAYS I think resistive versus non resistive plugs and their heat range is more important then what brand plug one uses and one primary cause of premature fouling is when we light duty use n parade thes old tractors around verus using them loaded and keeping the temp up to 180 and higher. Keep the ignition well tuned, dont run the carbs over rich, keep the temperature up and use a hotter range of non resistive plugs should help. John T
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