Do you really need 90psi to run a Stuart 10V? I would have thought 40psi or so would be plenty. My own 10V will run when blown hard by mouth.

Copper is very forgiving material, you are not very likely to have corrosion problems even after this time. (But make sure it is not limed up inside.) The inner firebox is a tube under external pressure, so is not a stable configuration. So if it was slightly out of round or dented at the start, the pressure will tend to make it worse. This is why fireboxes are usually stayed, or sometimes in full size, corrugated. For a model the usual approach is to make the material a bit thicker, but there is no easy way to calculate how much thicker it needs to be, and it is a bit late for that anyway unless you want to commit major surgery.

I would suggest that if you can, see if you can ease the bulges back to round, and then retest, possibly for a lower working pressure unless you are desperate for as much power as possible. If you read some of the articles about loco boilers, you will see that they sometimes develop a pincushion effect on the flat stayed surfaces on test…however if these are pushed flat again and retested they usually stay flat, since the copper has been work hardened. The same may apply here…the area around the firedoor will have been well annealed when the boiler was built

John

Woops, above is what happens when an extra paragraph is added and whole draft posting not proof read before final posting.