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The J plugs with the short ground electrode were meant more for when 30W motor oil was mixed in with the gas. It burnt dirty and left lots of carbon deposits behind. The short electrode was supposed to help burn off this carbon. With thermostat temperature controlled motors and newer oil designed specifically for outboard use, this style of plug was no longer necessary as the newer oils burned a lot cleaner and left fewer deposits behind. That’s when the J6-C’s came out to replace the J6-J plugs. They are the same heat range so I don’t think it would make any difference using them. Myself, I’d go with NGK B6S plugs as they are excellent plugs and I’ve had some bad Champions right out of the box over the years.