I run Autolite spark plugs in my Harleys. I always check resistance with my ohm meter, and make sure they have 0.00 resistance from the plug tip to the center electrode (small tip in the plug center). You want 0.00 resistance with breaker points or magneto ignition. I use the Autolite 45plugs which have a 13/16" socket hex in all the Harley 45 engines. They just happen to be a "Number 45 plug" and work in the Harley 45 WLs . . . a unique coincidence.
The Autolite 45 must be the perfect heat range, since they rarely foul, and usually burn perfect. A common plug you can find at your local auto parts store. But, here's the problem: A standard 13/16" plug socket is too tall to get them out with stock oil tank - if you ever need to change plugs, check jetting, etc. . . .
You need an offset 13/16" wrench like the one pictured above