This will be my proof-of-concept 1"/25 mm bore test engine for the single-piston fluidyne. The body of the engine will consist of two lengths of 1" copper tubing, closed at the top with a copper cap and joined with a brass regenerator fitting.
The portion shown in blue will be clear plastic tubing. It is not a tuning tube but, rather, a means of viewing the behavior of the engine and a way to allow adjusting the initial (static) internal air pressure. Once I'm satisfied with the engine's behavior, I plan to replace the plastic tubing with a T-fitting with check valves above and below to make a pump.
Before applying heat to the copper tubing, I'll fill the plastic tubing with water - and after the heat has been applied and some of the air displaced from the copper tubing, I'll add more water to fill the plastic tubing to the top.
I'm specifically interested in learning:
 The best air volume ratios between the (lower) hot head and the (upper) cold head. I can adjust the lower volume by inserting a small plastic tube down the larger plastic tube and feeding it up into the hot head airspace and either blowing in or sucking out some of the air.
 The best bore for the regenerator. I plan to begin with a bore that I think is too small, run a test and record the behavior - then enlarge the bore for the next test - and repeat until I recognize that I've gone too far. The bevel at the top of the regenerator is to minimize the amount of condensate that can be trapped in the cold head.
Follow-up experimentation will include varying the length of the regenerator to examine the relationship between the bore/length of the regenerator and the oscillation frequency of the engine.