Check this out, I got a set of brodix track 1s on my junk. They reccomend NGKs, so thats what I ran. then one day i decided to try a different plug, so i gets a set of autolites, supposed to be a direct replacement. I can screw the NGKs in by hand all the way to the gasket, the autolites would only screw in about two turns and stop, in every hole. The thread size, looks identical, but AINT. I'm not cross threading or anything else, they just won't screw in, and I aint going to push the issue. sure as the world if I do i'll strip those heads.