FYI, gents, that Stirling is Rudy Kouhoupt's design, and orginally appeared in his article, "An Air-Cooled Hot Air Engine." It's in volume 2 of his "Shop Wisdom" books, so if you ever thought you might want one of those books, this could be your excuse. I have both the book and the HSM article. The latter describes this engine as a project for a shop class. It has full plans as well as some good construction tips.
The original articles shows how Rudy built a little stove for heat that's pretty neat and then hooked the whole thing up to run a fan.
This one looks real nice compared to a lot of Stirlings, so it's been on my list to get around to one day. As these models go, this one didn't look like a "beginner" model, so I didn't want to attempt it as a first engine.
Cheers,
BW