Well, I´ve been fiddling around with the engine, done some slight changes, and come to the conclusion that it´s "good enough". I first made another crank disk, with even less throw, it´s now something like 13.5 mm. And cut some half moons away for balancing. The con rod is still a bit too heavy, have to drill holes in it. And round it closer to the M3 head.
Then I made a little bit longer "core" for the power piston, so that the Teflon mantle comes closer to the cylinder top. When I shortened the crank throw, it of course didn´t go all the way to the top any more. Now it does.
Then there was the overheating. To improve cooling, I cut the fins in the displacer cylinder some 1.5 mm deeper. There was (and still is) material enough. And what do you know, it now keeps on working for as long as it gets its rum ration. I´ve run it for over an hour non-stop, no signs of heat death. It just keeps on choogling. Even with the pudgy glass cylinder. And with a very brisk pace also. At least as much revs as the small engine does. What it needs though is copious amounts of spirit (=heat) - a huge flame. The displacer gets way hotter than in the small engine, but it doesn´t seem to be any cause for worry. The O-ring obviously gets even a bit hotter, but it´s still the original, and no signs (or smells) of it burning. Also the glass cylinder is holding its own, but it has started colouring red.
I´ve seen the same phenomenon in some photographs of German test tube Stirlings. Can´t say what is the cause.
I also changed the flywheel to the "original" ali thing. With the brass flywheel it would also run well, but slower.
And made yet another Teflon bushing for the displacer rod. That seems to be a sensitive point. I now left it a bit bigger on the outside, so that I really have to press it hard in its hole. And that seems also to tighten it around the rod better, it´s now quite air tight, and still very free to move.
There´s no Teflon tape in the power piston grooves any more, just the original mantle. What I´ve found is that oiling anything in the engine will not improve things. The Teflon piston should be as clean as possible. I often wipe it with a rag wet with alcohol, similarly clean the cylinder, wipe anything away from the displacer rod, etc. The Oilite bushing seems to slowly emit an oil film on it - no oil is definitely better.
So now I´m left with a number of Stirling "spare parts", and wondering what to do with them...
One idea would be something like this - but I don´t have 4 extra test tubes... Just three. (First picture)
Another nice engine would be like this, except FAR bigger... (see the match for scale in the Second picture)
Or maybe not a Stirling at all.
Thank you all for the encouragement and interest! I just might post some more pictures for this thread when I´ve had enough of blinging the lady, and mounted it to a proper base board.
