It's not only clearance at the sides, but have plenty of open area BEHIND the cylinder to allow the air to escape. Air getting by the cylinder means nothing if it can't exit the cowling completely. The air actually goes through the fins of the cylinder and that is what does the cooling. What goes past the cylinder between in and the wall of the cowling isn't doing anything. The area for the cooling air to exit the cowling should be about 2 to 1 of the opening for the air to get in. Make sure that the inlet opening is located properly for that job. This actions creates a kind of vacuum that makes for a positive airflow when the airplane is moving through the air.
Type at you later,
Dan McEntee