Help, i have a 1964 Valiant 225 3.7L and i cant find spark plugs with a 5/8" hex on it, everywhere i go they have the 13/16" hex, can somebody please tell me the brand and number (spark plug i can use) thank you...
the 13/16" hex are too big, socket didnt even fit....

The original plugs used a 13/16" hex (you would need the correct socket for them). The best plugs to use, though, do have the smaller 5/8" hex. They're this one, the NGK ZFR5N (stock number 3459). Remove the metal ring washer from each plug before installing it; the ring washers are not used on '63-'74 Slant-6s. Find more tune-up parts and technique suggestions in this post.

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Too many people who were born on third base actually believe they've hit a triple.

Make sure of the year engine you have. Does it have the aluminum spark plug tube inserts? If so, then Dan is right and 13/16" is stock but some aftermarket plugs use 5/8". Make sure to get the right plugs for your engine as there are two totally different types.

Lou

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To be more technically correct, make sure of the year of the head that's on your engine. It could have been replaced at some time in the past with a later year head, perhaps in order to "fix" a bad valve problem.

Roger

thank you...