I'll quote my HD owners manual... "Spark plugs must be tightened to the torque specified for proper heat transfer. Tighten spark plugs tp 11 - 18 ft-lbs per plug." I found the same reccomendation on 2 other 'name brand' spark plug web pages. I have tightened my plugs up to what I feel is pretty tight. Well, as tight as I'm comfortable with. When I check it with a torque wrench, ( use 2 different torque wrenches) it still isn't as tight as recommended. I'd appreciate any thoughts or opinions on this. I am kinda OCD when it comes to my bike and torques. The plugs are the only thing I'm hesitant on torquing.