Duncan, one other thing I understand the Aus code limits for steel boilers anyway is operating pressure which may give the road steam fraternity a lump in the throat! I can’t remember what the limit is and I don’t have a copy to refer to but I have 100psi in mind. Paul with the 6” Ruston down under can confirm I am sure. I have my Savage at 120 and it chugs along reasonably happily at 100 but primes at anything less than 80 whatever the water level if you demand a lot of steam so, if I were limited to 100 I would be close to the valves lifting all the time.
I wonder how any code would deal with the Marshall idea of no crown stays at all (rod / plate or girder) instead they pressed a Maltese cross shape into the firebox crown. Worked well enough in full size!
Paul.