Take a piece of cardboard about 8" long and 3" wide. Trace around the head of th engine on one end of the strip and cut this out. Place the strip on the fuselage so the hole is over the cylinder on the engine and tape the cardboard in place.

Repeat this for the needle valve and the exhaust (if the exhaust is needed).

Fold the cardboard back abd remove the engine.

Mount the cowl and screw in place.

Now fold the cardboard strips back down on the cowl and trace the hole openings onto the cowl. Lift the cardboard strips back up and put !/2" - 3/4" masking tape over these marks. Now put the cardboard strips back down and trace the hole openings onto the masking tape.

Now you can remove the cardboard strips and the cowl and remount the engine.

Next, cut the holes SLIGHTLY UNDERSIZE from the marks on the tape and trial fit the cowl. Enlarge the holes A LITTLE AT A TIME, trial fitting after each enlargement. Continue doing this until you get the openings to the size you need for a perfect fit. You will most likely have the cowl on and off at least 5 times for trial fittings before you get what you want for opening size.

The masking tape will minimize/eliminate any chipping of the gel coat on fiberglass or painted cowls.

After you have made the initial openings in the cowl, I suggest switching to a Dremel TAPERED stone for the adjustments/enlargements of the hole size.

The tape will most likely clog the stone, however careful use of a flat piece of metal will clean the stone. Secure the metal in a vise and LIGHTLY run the stone on the edge of the metal to remove the gunk.