Halcyon Valve stem nut (Bonnet nut)

[Ulfhedinn]

Any idea on the correct size to remove and then tighten this nut?

19mm is loose but works but not snug and 18mm to small.

3/4 is loose but works but not snug either.

 

[happy-diver]

What's a valve stem nut

 

[Ulfhedinn]

let me find a schematic

 

[happy-diver]

I have only got H manifolds and the nuts on those are bigger so can't check, they vary 19-21mm?
Bonnet nuts, metric RING spanners get them every time, a whack with a rubber hammer no marks