I posted over at DC.com as well and seems no one has an answer. Back when we did the 73 engine, in checking on plugs for the 400 4bbl, I came up with the original # of J11Y. I crossed it (then) to RJ14YC. I am about to get a new set and to my surprise, what is recommended RJ12YC which is the same plugs as my 67 - 383 4bbl. http://www.sparkplug-crossreference.com/convert/CHAMP_PN/J11Y So researching as I do, I realized the RJ14 is a bit longer. Not being a genius when it comes to this I have to question ... what is the difference between the two lengths and have I been using the WRONG plugs in my 73 400 4bbl all this time?