I just finished the layout board for my 3” X 6.5” boiler. The boiler uses 13 fire tubes and heats quickly. The burner is a home built ceramic unit working nicely at 5psi propane. The Clarkson vertical engine started this all off when I stubbed my toe on a box of rough castings. Some of the castings were hard as diamonds, but I used all of them. Running on steam for the first time, I see a few leaks needing work. The layout bricks are clay mini bricks I thought would look nice and provide insulation between the burner and the base board. I took all the engines off the bookshelf to give them a try.