People often remove the interior fan ducts to make it easier to work on the engine. If these holes are left open, a lot of the engine fan's airflow will escape through these holes instead of cooling the cylinders. I have observed without the caps, idling head temperatures of 380F. With the caps idle head temperatures of 280-320F with a smooth idle.
Comes as a pair of 2 caps, designed with a recess to fit a standard width hose clamp.
Hose clamps not included.
Made in Canada.