My manual tells me to torque my plugs at 18ft/lbs or major malfunction may occur. I tried to torque them at this spec, but it felt like I was going to strip the threads out....so I just tightened them by feel like I usually do. Do they need to be "specifically torqued"? In the past I have never torqued a spark plug, on a car or marine engine....any comments out there?...thanks.