THis is a simple and effective way to protect the plug wire from the edge of the cowl rubbing through it due to vibration...

Go to the nearest auto parts store and buy a couple of feet of 1/8" black rubber vacume hose. When you get it home take a razor blade or xacto blade and split one side of it lengthwise from one end to the other. Next take the split tubing and force the split edge over the edge of the hole you want to line, in this case around the cylindar head, push it down all around the hole and cut it off so the ends meet up. To hold it on just apply a bead of thin CA around on the inside of the cowl, gluing the tubing to the inside(not the outside) of the cowl. Think of it as one big custom made rubber grommet.