Nice engine Olli, you've really got the bug for these stirlings now!!!
Heh, yes. Like I told in the other thread, I bought 4 small test tubes, to have something to practice with, in case I break a few, but then I didnīt, and there are still 3 ready-cut tubes to find a use for. Here goes one, and Iīll keep the other two for spares. They WILL be broken some day.
From the sound of things, you should consider a lightening powered engine 
Arnold
Ehh, ok, considered. The answer is
NO. I emphatically hate lightning bolts. In Finland, I used to have a 30 m high (well earthed) tower for ham radio antennas, and there I got one hit. I wasnīt operating the rig at the time (of course, or I couldnīt be writing this...), but home all the same. I did NOT like the experience.

I considered building another similar tower here, but after the first winter thunderstorms, I preferred to live a little bit longer.
And an engine which may start for a few milliseconds, maybe once or twice a year, and hopefully when thereīs no one around to see it, doesnīt sound like my thing...

Yes, indeed, it looks (and acts) like a Robinson Stirling. That also means that it will run in the opposite direction to the two previous engines. Those run so that the flywheel top moves away from the cylinders. This thing should run so that the flywheel top moves towards the cylinders.
Once again, I did a mock up, to get some measures for the next parts.

And made the parts. The displacer piston rod is not yet to length, Iīll cut it later when I have the displacer made. Iīm planning on using a small ball bearing (from an old hard drive) to attach the power piston rod to the "balance" arm. I also made a small nipple to connect the plastic tube to the power cylinder.

And bored the power cylinder and made a piston. Itīs a blind hole, so it cannot be reamed. I let the boring bar (thickest I have) run several times slowly through the bore without advancing, until it didnīt shave even dust any more from the bore. The piston is a 7.2 mm ali bar which is pushed to a 7 mm hole in the Teflon bar. No need to glue it, I donīt think I could get it apart without breaking anything. When the air from the hole in the PTFE has been pushed away, thereīs an almost perfect vacuum holding the pieces together, if you try to pull them apart. And of course the ali "heart" will stiffen the Teflon for turning it into size. The cylinder bore is 13.7 mm, that seemed like "enough". The engine in the drawing has a 12 mm cylinder. This engine will have a shorter stroke, however.

Thereīs also need for a tapped hole in the cylinder side, to let the pressure in. And BTW, it is an excellent idea, NOT to push the piston all the way down in the cylinder, before drilling this hole. It can be a real PITA to get it out, if itīs a reasonable fit (and especially if itīs a bit too tight... Donīt ask me how I know...). With the hole, you can blow it out. In the drawing, the said hole is rather high from the cylinder bottom, Iīd prefer to have as little "dead" air space as possible in the cylinder. Thereīs more than enough in the connecting tube anyway. So my hole goes to the small cone that is an artifact from the drill, from boring the initial hole.

The piston entering the cylinder. I am holding the tube plugged, and it wonīt enter any more. If I let loose of the tube, it will start descending, and hit the bottom some 25-30 sec later. It may be too snug, but thatīs easily remedied.

And another mock up. The power piston rod has no big end yet, itīs just tied to the balance arm. As you can see, the power piston and the displacer are not aligned, as in the drawing. The drawing has a double power piston rod, that goes on both sides of the displacer rod. That might be a good idea, but Iīm trying to get away with just one rod. It will need either a better guidance in the balance arm, or a little more slop to compensate. Of course Iīll try first the slop...

The displacer proper once again seems problematic. I still donīt have suitable ali tubing, or any smaller test tubes (as in the drawing). And I donīt think I have any suitable electrolytes either, this time. Brass tube might be one possibility,but Iīll figure out something.
Thatīs about the size of things as of writing, there might be some days before I come up with the displacer.
Thanks for all the comments, much appreciated!
