Given that you're an SBA member now, give Kevin Slater a call: as SBA President currently, AND owner of a boilermaking company, he has lots of contacts, and may be able to suggest something. Incidentally, I think his waiting list for new boilers runs to something like 2 years.

There's nothing wrong with being slightly bonkers: steam involves eccentrics everywhere! Conventional domestic immersion heaters are not designed to take any significant pressure, and I don't know what would happen if they were subjected to it. However, if you ran one through a 1:1 isolation transfo...

Going back to the original posting and request, drill-service.co.uk do a range of specialist drill bits including left-handers. I'm doubtful whether they'll have reduced shank versions, and some lathe or grinder work may be called for if you really need that, though a reducing adapter may be easier,...

Don't try and use hand taps for this: they may fatigue and break off with that many to do. As Bart says, though, with a machine tap, which will probably have spiral flutes, it's a doddle. Getting the tap etc. properly at right angles to the work piece is harder (for some reason) than with a hand tap...

In my view, go with Swagelok Fittings, very reliable fittings. Agreed. Anyone who has done one of the approved swagelok courses on their fittings ought to agree as well. They are a form of compression fitting though, so is there some let-out clause for using them in boilers? There probably ought to...

I haven't done it myself, but model locomotive builders often use brazing for their boilers, and they tell me that getting everything hot enough is really rather difficult, even on boilers for the smaller models. I've done a fair bit of vacuum brazing of industrial pieceparts, and that makes a lovel...

Don't be afraid to experiment. But don't go blowing yourself up! Surely the second half of that line can be reversed when politicians are the experimenters? Willingale Tubes are possibly the main suppliers of finished boiler tubes in the UK, and they go to great lengths with their tube bending syst...

Wicksteeds (https://wicksteed.com/wicksteed-products/tube-expanders/) make a big range of tube expanders, and while they aren't cheap, you only normally need one for a boiler, and compared with the cost of a new boiler they're not expensive at all. There must be a comparable manufacturer in the US w...

With aluminium it is SOMETIMES possible to convert its surface into a hard anodised layer, which is wear resistant and can be suitable as a cylinder surface, but not all grades of alloy will allow good strong anodising, and in any case finding someone to do it properly may be more difficult and expe...

Now you've got all those stresses to either get rid of or live with. I recommend marinading the welded unit in something which will produce a thin organic film on the steel, having dissolved a bit of grease beforehand. Something around a 4% v/v solution of methanol in pure water would do. If need be...