Leaking Sight Glass (Video Link)

I would try Tightening it slightly first.
If that doesn't work, replace the crush washer. 

If tightening doesn't work take it out and apply a sealer on the threads.

Any idea on a part number for the crush washer? Im not sure where to look for that sort of information.

@ Roger: do you think Loctite 567 would work? Ive got some leftover from the brake line fitting assembly I did earlier in the build.

567 should be Good.
Try tightening it a Half-Flat first.
Is this on the Expansion or the Overflow Tank?  They are two separate items.
What Aircraft is this on?
Is this tank a Rotax Part?  Part Number ???

Hi Bill!

RV-12iS. Rotax 912iS. 

This is the Rotax expansion tank as depicted in the attached picture below. Not the overflow tank. 

I tightened the sight glass as yall suggested, but havent had the opportunity to test it yet. The key thing to see in the video I originally filmed, is that the leak is not coming from the screwed in sight glass, its coming from the inner ring that the sight glass screws into. The inner ring that seats into the expansion tank itself. Very strange. 

Again not much in the publications about it, but im hoping to avoid having to buy and install a new expansion tank!