Starting in the mid 60s, thru the 80s a majority of engines used Surface Gap Plugs. They were touted as being less prone to causing Preignition than Conventional Plugs as they were a very Cold Plug. And though that could cause Fouling, it was advertised that the very High Voltage of the then current Ignition Systems, would still fire the Plugs. I had them on several outboards and never had any trouble. My 84 Merc 115, had over 1500 hrs on the original Plugs when I traded it in 14 yrs later for a 98 FICHT. That thing went thru dozens of Plugs the 4 years I owned it. But since OMC was paying for them, I wasn't too concerned. My Dads 1980 Evinrude's Owners Manual made reference to a Conventional Plug that was recommended if the engine was used at very low speeds continuously, otherwise the Surface Gap was to be used.
