The shell expansion would presumaby be matched by the tube-plate expanding a similar amount. I haven't seen provision of a gap as an issue in boilers I've made. More like getting the tube-plate a reasonably close fit in the shell. If a capillary gap was seen to be needed then filing a very shallow taper around the tube-plate flange would suffice in my view.

Not sure of the wisdom of a drive fit ? If heating is correct then the capilliary and surface tension will pull the tube plate into the centre and maintain an equal gap all round, all be it only may be 3 or 4 thou. The harder part is to hold the tube plate in place in the shell, there are many ways. If the soldering is sound there will be a very small fillet, the joint will be filled by solder and therefore steam tight. Good luck Noel.

Yes, maintain a slight interferance fit. There will be plenty of so called gap for the solder to penetrate the joint. Whatever you do dont try to manufacture a gap. How can I be sure of this? I make boilers for a living and have never had a joint faiulre – ever (having made hundreds of them). Anyone wanting to learn is very welcome to come visit me as Pendle Steam Boilers in sunny Burnley.

The flux is Stay-Silv Black by Harris Products apparently – I found their website here. Don't know if it's available in the UK though. She mentions it contains boron to extend the temperature range.

Links to https://www.harrisproductsgroup.com/en/Products/Alloys/Brazing/Fluxes/Stay-Silv-Black-Flux.aspx

Rob

strangely its not on the cup website – but thats where i get mine from – I bulk order so dont go via the website