The plan for 622’s boiler is somewhat experimental. Because the engine is a split frame design, and the running boards are connected to the frame, the boiler needs to admit both polarities too. At the same time, these little engines are mostly air, and so, we need to fill the boiler with as much weight as possible.
I considered casting a custom lead weight and fitting it inside the boiler. But the shape turned out to be very complex, and I wound up having a shell printed in steel, with cavities for the denser materials.
After no less than three sessions with The Boy and His Blowtorch, we managed to get the boiler full of lead and Pinecar tungsten weights, without melting the steel casting. One side of the boiler was insulated from the hot lead with kapton tape, and this did not stick to the molten lead at all. That enabled me to see inside out initial attempt, and find there were many air gaps. The second attempt yielded a puddle of molten lead building up on my vice while two befuddled Gourleys merrily continued to feed lead into the cavity from the top. The third time was the charm, however, as revealed upon inspection.
While The Boy and His Blowtorch were hot to trot, we also annealed the flanges on the running boards that engage with the boiler. These flanges were one of the successful ideas from #10, and after I had spent an hour or more fiddling with them, they worked remarkably well, fitting perfectly into the recesses in the weights.
As I was fiddling with the flanges, I folded the temporary end cap of the running boards down. Interestingly, this enables the boiler to sit almost level when left on the table. When I was building #10, I had a special cradle to hold the boiler level while I worked on it, but a built-in cradle is so much easier to use. The next locomotive I design will feature such a cradle as part of the running board assembly.
I stood the assembly vertically in my vice, recently cleaned of lead, and epoxied the running board to the boiler weight. It was only after the epoxy had cured and I sat the boiler on the frame to see how it looked that I discovered to my dismay that I have etched the frame backward! The boiler will short out through the screw into the cylinder saddle. Well, it still looks pretty cool, even if there is no way it’s going to work.
Pembroke:87 
Do you know how you got the frame backwards? Was it in Onshape, or when you made the etching file?
Could you use a plastic screw? Thinking about those plastic screws that hold car interior parts on.
Craig
Hi Craig, it was when I made the etching file. OnShape doesn’t distinguish (or didn’t) between convex and concave bends when producing the flat shape. So I must have mixed up the front and back of the pattern at some point. I really should have printed the pattern and done a test run. Well, a lesson for next time!