I've been running the numbers for the RRS heavy mike I'm building and finding differences between what is drawn and what I calculate. I'm not trying to second-guess the draftsman, just how to best set up my blast nozzle/petticoat. I have done the calculations, drawn the 1:6 and 1:3 cones, and found significant differences making me wonder if I am doing something wrong. If you don't mind, I'd like you to double-check what I have figured. Cylinders: 2.375" bore == 0.339 (say .340) nozzle opening Stack opening at top of taper: 2-17/32. Petticoat diameter: 2-1/4 (straight) Height of nozzle as set by 1:6 cone: 1" from smokebox bottom. Gap between nozzle and bottom of petticoat as set by 1:3 cone: 4-5/8" Picture Legend: Blue pencil: as-drawn nozzle height Screwdriver: as-drawn petticoat bottom Green pencil: as-calculated nozzle height Knife: as-calculated petticoat bottom Black pencil: top of stack. yellow cone (1:3), white cone (1:6). I first lined up the 1:6 cone with the top of the stack, then lined up the .340 nozzle line on the 1:3 cone with the 1:6 line. To make the the blast nozzle/petticoat design match up with the formulas, I would have to make the petticoat tapered, extend it's length to the centerline of the smoke box, and shorten the nozzle height to about 1". We'll see if I actually to that.