
I've been using 18mm Champion spark plugs like this in my 75 year old Harley for over 40 years. I bought the candle catalog 516, also "D-16". I keep coming back to this candle ever since it was cataloged as "UD-16" in the 1970's. My old Harley uses 18mm oversized spark plugs, commonly known as "tractor plugs", which have been used in farm machinery for decades. . I liked to find them in boxes of 6 and fairly cheap. The price was right!

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