CM-6 plugs are factory checked at .018". I've never seen a real reason to change them. Gapping to .017" saves re-gapping every year or so. That's correct, I said every year or so. Most people will easily get two or three years out of a plug if they tuned the engine close to right. Champion plugs seem to like .022" to .025". NGK Iridium plugs are gapped according to the amount of energy the ignition can deliver through the plug. Weak/average ignitions at .018" and hot ignitions can use .025" to .032.

Easy way to check what your ignition can handle is to experiment with wider and wider gaps. When the engine starts crapping out or loses performance, reduce the gap.