AV8TOR, The 2 plugs I got that are OS 4cycle has a letter H (I think) stamped on them. I wonder why they do not stamp "F " on them, It is hard to read the small letter on mine. The one that was brand new really made the engine run good. maybe the H means HOT! Thanks, Capt,n P.S. Don,t get fuel spray from carb get on plastic eyeglass lens. It makes them go bad. Don,t ask me how I know!