Step 8: Finishing Up
First make a 90° bend in the piston rod 20.5mm from where it enters the end of the piston. Now insert the piston into the cylinder, then simultaneously push the cylinder pivot rod and the cylinder rod through the body and the crankshaft respectively. You will likely have to turn the crankshaft to make the hole for the cylinder align with the bend in the cylinder rod.
Now you just need a way to hold the cylinder tight up against the body. I used a spring from a retractable pen held in place with a screw on "prop-saver" from a model airplane. I also found just putting an elastic band around the piston and body worked quite well... if not better than the spring, this option is also much easier to remove than if you glued a cap on the cylinder pivot rod. An elastic or two should really be all you need here.
Lastly you need to connect one of the top two holes on the flywheel side of the body to a compressed air tank or pump. To do this I just glued some plastic tubing to the body; just do whatever works best for you. This motor works quite well for me on ~15psi. Turn on the air and give the flywheel a quick flick and the motor should start, if not try flicking it in the other direction. If you line it up just right the motor will even start without you flicking it. Switching which hole the air goes into will reverse the direction of the engine's rotation.
If your motor does not run first make sure everything spins fairly easily when turned by hand. If it doesn't try figure out what is stopping it. Next make sure that you did not accidentally get glue in any of the holes that supply air to the motor. Lastly try running the motor on higher PSI. Other than that there isn't much more I can suggest. You can try posting any questions in the comments, I'll see what I can do but I'm no expert on the subject.
Over all I am very pleased with how this little motor came out. It runs very smoothly, looks good and was really easy and cheap to build. Some possible improvement include; the use of bearings to reduce friction, waxing the wood where it rubs together, or replacing/coating the wood with a layer of Teflon which would greatly reduce friction and help with the air seal. You might also try making the air input/output holes a little bigger and upping the PSI for more speed.
If you have any comments, questions, or suggestions please don't hesitate to put them in the comments. I'll do my best to respond to them.
I hope you enjoyed my instructable and I wish you a successful build.