ZDZ40 with the scratch-built gas pattern (ama plan). I recall the plan calls for light tube used for heli tail boom. It worked pretty well. If you have the flange for the exhaust port, find some mop or broom with steel tube handles. At some point of time, I examined one such tube and found it is extremely light and can be map gas soldered to the flange. The coupler I used is pretty short (1" or less and no metal insert).