Thanks for these replies.
Ron, I'll be using a gasket on either side of the mesh & working sealant into the holes in the mesh as you describe. I think I might try Hylomar gel this time around?
Ian, thanks for the info on the studs. I just looked at a spare crankcase which has blind holes. It makes obvious sense to have open threads sealed properly. In peoples experience, is it hard to remove these studs with the motor in the bike?
Scraping the crud away, I've had a better look at my crankcase casting & the left front stud hole runs into the interior. I might redo that one come oil change day. Judging by this example no other holes could be open on any similar casting, which I presume means all of them?
Bob,
I did this with the motor in the bike. One stud had iffy threads and would not work with the 2 nuts method.
I had to weld a nut on the stud and got it out that way.
4 new studs and seal on all threads and no more leaks. 2 years.