Just about all 1950s OMCs used Champion J6J plugs. Today’s offering is a Champion J6C. The C stands for Copper electrode. When you get into the 1960s, the bigger motors (40 HP up) mostly used the cooler J4J.
good thing is just bought a bunch of J8C’s and J6C’s got 14 and 24 respectivly for the H10 as i think my FD10 takes H10’s could i sub in j8C’s as i see they are between J7J and J8J and people have for other motors said to sub in j8’s for them.
I did notice the H10 & H9 plugs have (t) note that says they are 7/16" reach threads. That might make them stick thru the head too far? You would not want the edge of the threads in the combustion chamber. I agree they are up the heat range a bit too far! Might be good for trolling only?
Easy enough to make "C" plugs into "J" plugs. Just file the ground strap back until it only covers half of the center electrode. You then have the old "J" style plug.
Your motor of course and you can do what you want, but they are not recommended for any of the 50/60s OMCs. I would toss them out. I pop old plugs in some motors for storage if I don’t have proper ones around. THat may be why you found them.
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