This little engine, a 2x scaled up version of the "Milly" from the Sherline book (and elsewhere ?), was built by my 19yr. old son and I last weekend as our first engine project. The original plan was to tackle a large Stirling fan but this was a far better introductory project for him (new to the shop).
We've got about 6 hours into it. It's 6061 and stainless. A drop of light oil before running takes care of the parts that should be brass or bronze The little thing seems like a screamer, around 1,000 rpm on a 20 psi supply. I realized that scaling it 2x would require 4x the air volume but now it's big enough for me to see (about 2.5" tall) and for two guys to work on without getting in each others way. Den