5 heat range for NGK. I'm 10:1 340 and run pump gas 93 with RHS iron X heads. 3924 is autolites 4 heat range. Both run clean in my motor. My used plugs from my 67 go in my 73 340 j head street car. The 5/8 plug gives a little more socket room with headers.
I won't run champion in my lawnmower. Years ago a set I bought had 3 bad plugs.
