To get a very close cowl fit. First take off the cyl head.Loosen all the bolts 1/4 turn at a time until they are all loose and remove the head, do not lose the head gasket. Get some card board like a file folder or construction paper. cut a piece about four or five inches long and two and a half inches wide . Tape it on the fuselage behind where the cowl will screw on and so it covers the cylinder. Take a sharpie pen and outline the cyl onto the paper. Trim out this hole so it fits the way you want it to over the cyl. Then slide your cowl on to where you will screw it on and trace the hole of your cyl onto the cowl. You can do this for anything that protrudes out side the cowl. (The same goes for finding where to drill holes for attaching the cowl.) Make sure before you cut to have the cowl positioned over your crankshaft so its even all the way around. Take your time measure three times cut once. It really helps if you have a dremel with a grinding point. After your cowl is fitting perfectly. Reinstall your cyl head. Make sure you torque it to the same value as what you removed. Tighten all bolts in small increments in a criss cross pattern. The biggest thing here is to make sure you don't over tighten one bolt more than another. Good luck and take your time.