No luck solving this so far.

Probably fuel (ethanol) related.

The plastic is way indented between the threaded inserts.

Thicker o-ring didn't help.

Any ideas?

If it is ethanol swelling, you can drain the tank and let it dry out for a few days. Swelling will usually shrink back to size.

I have stopped using ethanol fuel in my bikes.

As I recall, older shercos came with a gasket on the petcock. Might try combining the two to get a good seal if you can find the gasket.

Other thoughts include trying a good RTV sealer like three-bond. I think it is fuel resistant, however it is not alcohol resistant, so must cut the alcohol out of the fuel.

I do not know if it is possible to find or make a thicker seal ring of square profile that may work.

I'm using no alcohol fuel (100 oct avgas).

But, Copeski...you're the 2nd one to mention a square section seal...the first being a hydraulics guy who spends a lot of time dealing with seals.

So far my best fix is a carefully-sliced bit of 5/16" fuel line.

Seems to be holding...

I've had the same problem with a 2006 125 so I used a rubber grommet to seal it but then found out it is leaking from one of the treads in the tank as well!!

I'm going to attempt to remove the threads and manufacture a plate to go inside the tank for the petcock to bolt to. Plus a little bit of that sealer copemech suggested!

Not sure what keeps those inserts in there.

I wonder if the tank is molded around them...

Ryan would probably know.

Lol the tank USED to be moulded around the screw inserts in mine but over time it has cracked around one of them so I have removed it completely ready for a plate inside instead. Hope it works as there's no going back now!

There is a putty made by one of the well-known companies (Loctite?) that is for sealing fuel tanks.

Probably your best bet for keeping the plate sealed.

I used some to fix a fiberglass tank and it's been holding.