What plugs can I use in a 1930 D?
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The side valves are pretty forgiving on spark plugs, but the early advice for a high compression engine was to use a Harley No 4 plug in the hotter running front cylinder and a No 3 in the rear. The Harley No 4 plug is equivalent to NGK A7 or Champion 4COM, 5COM, K9 and D9JM while the Harley No 3 matches against the NGK A6 or Champion 7COM, 8COM, D10, D14, D16, D16J, D21, D23, D9, D9J, K11, K13, K15J, UD16 and UK 10 plugs. A modern version of the Champion D16 is not hard to find, and you could start with a pair of these. -
Thanks, will try some D16'sComment
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