This is a question which pops up from time to time; there’s a hole in the side of each head in about the middle, under the tappet covers. This is the case on all twisted twins – CX500, CX650, GL500, GL650. What’s it for?
This is a drain hole for the spark plug well – without it any water which got under the rubber cap would sit over the spark plug with nowhere to go. It should be clear so that liquid can flow through it.
If there is oil leaking out of the hole in the side of the head chances are that the rubber gasket which sits in the middle of the tappet cover is leaking, allowing oil to flow down over the spark plug and eventually make it’s way out the drain hole and down your cylinder head. Double check the positioning of your circular rubber gasket and if necessary replace it.