Stirlingengineforum.com

Nice machine! I think copper is underestimate, its easy to solder and its look really good. I found that it is quite easy to solder stainless steel. https://twitter.com/BenKrasnow/status/1251262459517665281 By having a stainless hot end, you should avoid any defiormations of the hot end if it gets t...

I would probably bet a bit on the more efficient stirling engine. You can get efficiency without using modern technology. See: https://orca.cf.ac.uk/54057/1/U585566.pdf 1840 Separated regenerator (s. 46 in pdf, s. 17 in book) 1845 Cycle under pressure (s. 46 in pdf, s. 17 in book) 1880 Air preheater...

I also thought along those lines but have not able to find a reasonable engine with 90 ° angle between the cylinders.
An appropriate engine would be V4, but the ones I've seen have 180 ° between the cylinders.
A V8 could work but is big and relatively expensive here where I live.

pressure is the same on both sides. No, it is not the pressure will have a little difference between the hot and cold sides. It has to do with the regenerator slows down the gas. The difference is not huge, but enough to give the displacer enough energy. By the way, I do not think this type of Stir...