![](/forums/profile/get-photo.asp?memberid=7567&type=profile&rnd=604) Expert 5K+
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I thought for sure that Powerflite (Nathan N.) would have your answer by now because I believe he is running a 331 in his 56 Plymouth.
After looking through the 55 Chrysler Factory Service Manual (downloaded from MyMopar as a pdf), I could not find the spec for the OE spark plug.
BUT the FSM did say that it should have 3/4" threaded, that the washer should NOT be used and the gap should be 0.035".
I checked the reach on your N14Y and it seems to be the needed 3/4" so that is good.
Not sure about the heat range.
I have posted this before and I do see a 3/4" reach N18 and N16Y (the 3/8" reach stuff relates to my stock 315 D500 engine):
Hope that helps.
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Edited by 56D500boy 2024-05-09 5:42 PM
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