Hi all, I'm a first-time poster and I have a "Head Scratcher". I'm a relative novice, not a true gearhead. I'm building a 1969 318 to swap into my 1974 Plymouth Valiant slant 6. I ordered spark plugs - RN14YC, but the diameter of the threads (14mm) is too small. The plug falls right into the threaded hole. I went to a parts store, and they confirmed the plug reference number and brought out the same plug I already had. The guy cross-referenced it, and came out with a 18mm threaded plug, but...
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