The original article on building County Carlow’s boiler says use ‘one of those high silver content solders’ over the stayheads outside and then over the nuts and thread ends inside with the same stuff.

IMHO this suggests the boiler is unfinished and that the missing nut is needed and then the inside stays need caulking with Comsol or similar.

ME Jan 2 1970

You need ingenuity! A method I have used is to find a length of rubber tube that the nut fits tightly, and use it as a flexible drive to run the nut up the thread. This can work even if you have no access straight on. Getting it started may need something to reach in and guide it initially, perhaps wire with the end coiled to fit round loosely. If you can get the nut started and most of the way home, find a spanner, small ratchet or what have you to finish tightening from afar.

Edited By Macolm on 21/03/2023 17:46:21

Impossible to say without having access to the full constructional details and doing the calculations. (Or CAD modelling the boiler and letting the software do it.)

My reaction is an unstayed boiler is unsafe at any working pressure, don't risk it. Stays are specifically inserted to protect the structure at weak points and are therefore essential. That part of the boiler needs to be right. Fix it as the other answers describe.

There are many examples of full-size boiler explosions due to stay failures. Corrosion and other problems can be hard to see and metal fatigue is a non-obvious problem. Each time the boiler is steamed the unstayed section of boiler plate balloons slightly. The in-out cycles strain the metal that bit more every time until it comes apart, most likely at full pressure and with spectacular results.

Dave