Last night on the show+tell, I talked about the little steam engine I built last fall. It’s a horizontal single-cylinder, 1cc displacement, with a scotch yoke drive mechanism. It’s a combination of manually-machined and CNC-machined parts from aluminum and brass stock. Everything but the mounting bolts, the shaft bearing, and the scotch yoke dowel pin were made by me. It runs at about 3500RPM at 30 psi. Below is a short video I made of it running:

You can also check out this little stop-motion vid I did of the drive mechanism:

Finally, if you want to learn more about how scotch yokes work, and see how they differ from the typical crank arm and connecting rod linkage, you should check out this excellent demo at Wolfram Research, which illustrates the accelleration and velocity curves of each.