Posted for "Pat and Sue Brunner" <n98pb@sbcglobal.net>:

 The opening width is exactly the width of the cyclinder cooling fins, about
 2.3" and its about 4" total length with a tapered area top and bottom.
 essentially it allows airflow from the high pressure side, around the flat
 spot of the cyclinder and out the bottom with redirection across the bottom
 of the cyclinder.