How on earth do I build this boiler for my Fire King ?

I'd make the shell from 4" 16g and the firebox from 3.5" 16g. Then you roll a ring from 3/16" square to make up the gap, a lot easier than making the conical firebox

Just my opinion Bob, but I don't think having the firebox inner and outer shell meet at a sharp junction with no foundation ring is a good idea. Depending on the water you use, there may be a lot of sediment buildup at the lower end of the boiler. If it is a small space tapering to a point, it will fill quickly and you may get plate overheating if there isn't water between the shell and firebox inner because the space is full of sediment. A foundation ring provides a) a water space to keep plates cool / transfer heat, b) a wide ledge for sediment to fall on, and be shifted / blown off during blowdown. Food for thought.

Edited By Jeff Dayman on 08/04/2021 19:31:49

Yes - your modifications all looks good, but I note Jeff's observation on taking the firebox wall out to the shell, without a foundation-ring. Looking at your second drawing the annulus is finely-tapered and as Jeff says, could silt or scale up quite easily. Thinking about it further, the full-size boilers made without a foundation-ring had a step formed round the firebox shell so the water-space stayed wide for its full depth.

If the fuel is to be spirit or better, gas, you are not going to need to open the smoke-box very often, but if the rings will be prominent, visible features on the finished model, then fit them even though dummies. .Faithful external details, even if not functional in miniature, greatly enhance the complete model.

Thanks chaps for all your useful comments, and sorry about the delay in replying. SWMBO insisted on using the relaxing of lockdown to use the opportunity to go off in our caravan for a much needed break. Anyway, I have made a strategic decision which may not meet with the approval of some. This model will only run on air. Firstly, it makes the construction simpler and secondly, it means that I can show it in my display cabinet along with my other models. As I said earlier, if it was good enough for Cherry Hill then it’s good enough for me.

Bob