You could drill/dremel it out, and epoxy in another knurl nut ?. Contact the manufacturer to buy the nut. Shouldn't be too hard.
^^^ then youre into the plastic so dunno why you would go that big.
It still has some thread so i would go tape like 'sloshin said.
If not, then tap to M6 as at least it it a windsurf standard.
Care - the insert is not bonded in well so dont go nuts on it
7/16 unf tap is 11.13 mm major diam and 9.73 root diam. Assuming that is a 10m1.5 pitch o-ring plug that is stripped.
The od of a 10m1.5 helicoil is about 11.7mm.
Thread tape will not have any shear strength needed to clamp an o-ring.
The torque required to tap a new thread may exceed the bond strength.
A new (sharp) tap and a careful hand would help.
Any consideration to any windsurfing standard thread type is not relevant.
Their is a risk the od of the brass insert may not be enough to tap to 7/16.
7/16 unf tap is 11.13 mm major diam and 9.73 root diam. Assuming that is a 10m1.5 pitch o-ring plug that is stripped.
The od of a 10m1.5 helicoil is about 11.7mm.
Thread tape will not have any shear strength needed to clamp an o-ring.
The torque required to tap a new thread may exceed the bond strength.
A new (sharp) tap and a careful hand would help.
Any consideration to any windsurfing standard thread type is not relevant.
Their is a risk the od of the brass insert may not be enough to tap to 7/16.
It seems to be a finbox screw thread that is the problem, from the picture, not an o-ring plug.
I was thinking a helicoil would be an option. Fairly easy to fit.
Stainless(316) Helicoil and brass/bronze=corrosion over time, less noble mettle =galvanic action.