they can be fitted with lower clearance to the cylinder, but at the same time they don't like spark knock and can easily damage them. so for me i have to try and play my going lean as careful as i can. lean wise i will be running my engine for a month or so at 14:1 then check the plugs see how they look. then bump it down to 15:1 and then do the test again then check again. i don't know of any way to tell spark knock as from my research on it everything i found says you can have spark knock and not even feel it 