Hi Bob,
I'm afraid, I didn't do a very good job with the sketch, but I hope you can still see how the tool works. To get the exact length of the tube, measure the distance of the holes in the boiler from center hole to center hole. Transfer these measurements to a piece of hardwood. I usually use beech. Then drill the holes in the diameter of the glass tube into the wood. Cut the wood exactly in the middle of the second hole. Bevel the edge there. Don't forget to bevel the edge of the other hole towards the second hole as well.
I hope you understand my description. If not, just ask.
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