Disclaimer: This is my first ever 3D project (I just started learning Cinema 4D) so please be gentle on the critiquing...

I built this design (and had it working) in my high school metal shop a long, long time ago (well not that long ago...) and thought I'd recreate it for you folks. I still have my original one I made somewhere, so when I find it (...in a box in the garage...), I'll post some real pictures of it later. I'm also including my original C4D projects if anyone wants to play with the design.

It's a very simple design but I haven't tested this exact 3D model yet so keep in mind it might need tweaking of the positions of the intake and exhaust ports and maybe some dimensions. Precision of the cylinder/piston and the two surfaces of the cylinder/block are the most important to make this work.

My instructions call for metal parts because that's how I made my original one, but I think with some machining with plastics (you need tight fits on piston/cylinder and other stuff...) I think it might still work.

The way it works is simple:

1. Down stroke: As the crank moves clockwise, the piston tilts the cylinder and lines up with the intake hole and receives pressure.
2. Up stoke: With the momentum of a heavy flywheel, the cylinder then tilts to line up with the exhaust port so the pressure in the chamber doesn't stop the upwards motion.

..That's it. Let me know if anyone actually makes one and gets it running. Hope you enjoys it!

Here's a video I found with the same concept engine: http://www.youtube.com/watch?v=vcaFWg-FvbQ