Traditional Boiler Rivets Vs Welded Rivets.

Having replaced some of the stays on engines being mentioned, Bluebell ones, I hope I can shed some light.

The SR used monel stays quite a bit, 30541 has them. They are the same overall stregth but thinner than a copper stay. This means you use the same pitch for the holes. You then have the option of going up to the larger copper sizes without changing the strength/stiffness of the staying. Although by then you are probably getting into thinning plate issues.
Monel stays DO break but it is rare.
Monels are caulked on the outside and caulked and nutted on the inside. The SR riveted them over in places too.

Replacing stays is not difficult of itself apart from being hard/tedious and skilled work. The problems arise from the location of the stay. Backhead? No prob. Drill out, shell out, tap hole, fit stay and rivet over. Firebox side? Tanks off ( if it's a tankie), cladding off then as before. If they are below the brick arch, then on a lot of engines you have to lift the boiler far enough to get at the outside. This goes for the throatplate too if there is a stretcher in the way. There usually is. So disconnect steam pipes and injector pipework, undo most of the pipes in the cab including vac brake gear, maybe take the reverser off, lift off the top of the cab, plus other stuff I've forgotten. Then lift the boiler far enough to do the work.
You do have large enough cranes and somewhere to do this don't you?

In the old days blind stays were used. A special tap was used from inside the firebox and a sort of self caulking stay was wound in. Not seen it done but I have handled the taps. Bit a fix really, only to get you out of trouble.

Russ.

PS If you want a really boring job, try doing patch screws. Everybody wants to drive, fire and grice these things but there is not a big queue for the boiler tools!