Leaking Sight Glass (Video Link)

I see the leak as coming from the backside of the crush washer.
The sealing surface on the tank appears to be very coarse.
You might try smoothing that surface to give the Sealing washer a fighting chance.

I have a collection of 912is and ULS Parts manuals in my archive.
I can not find any versions with a sight glass.   ???

What is strange is that there is a Sight Glass at all.  Who added it?
The expansion tank is expected to be absolutely 100% Full of Coolant at all times.
Unless the sight glass is at the top of the tank, by the time it showed any coolant level, you would be on the verge of circulation a partial mix of air.
If you ever see any level in the sight glass, Either the overflow tank has long been empty or something is very wrong with the cooling system.

I use clear tubing from my Expansion Tank to my Overflow Bottle.
I expect that Clear Tube to be completely filled (No Air) at all times.
It is a Pre-Flight Checklist item.

To Follow up on this issue.

I tightened the sight glass about 1/2 a flat rotation. reapplied torque seal and cleaned off to check for any future leaks. 

After 4 additional flight hours I haven't seen the leak come back. Thanks again to Bill and the forum for the excellent advice!

What on earth is the point of this sight glass??

What version of the MML was that diagram in?

Just as a point of reference, I received my RV-12iS powerplant kit May of 2019 and it’s got the sight glass too.

I see lots of the sight glass expansion tanks on the newer engines. I haven't had one leak though.