I cut the inlet air hole and welded 2.5" pipe in an angle so the air will swivel and create more heat in the center. I kept the pipe 2" inside that will be the thickness of the refractory in the furnace. I found my welding magnets very usefull for holding the pipe in place. After getting all my welding gear ready the pipe was perfectly welded into place. Small tip: when placing the cylinder on the side place 2 wood battens under it to prevent it from rolling aside when using the grinder on it.