Edit:
I'll try and find a pic, definitely seen one somewhere.
But, looking through the hole, at the bottom is a little round recess where the tip of the plastic gear "slots" into, the gear rest on the gear inside the casing and spins...
If you did not have any securing on top, wouldn't there be oil spilling out everywhere? Was nothing in place on the top, just the hole?
Edit2:
Thought it was on broke...here this should help:


