https://www.youtube.com/watch?v=5oGbH8k0wpY https://www.youtube.com/watch?v=Qg2YyzafKsM&t=29s A few months ago (February, 2017) I posted a normal and slow motion video of a four cylinder air engine on my YouTube channel (https://www.youtube.com/user/popzct0214/videos). After experimenting with the design, given its weight and air flow requirements, I put in on the shelf and decided not to publish it. Since then, I've had a number of requests for the STL files, so here they are. Remember, this engine is experimental. so be prepared for precision filing and fine adjustments if you decide to print and experiment with it. This design utilizes a spinning rotor to direct air into each of the four cylinders. The rotor contains four holes, spaced equally along the length of the rotor, and rotated 90 degrees from each other. "Gear Crankshaft.stl" and "Gear Rotor.stl" are designed to maintain synchronization between the rotor and the crankshaft to make sure air is delivered to the cylinders at the proper time. The design depends on cylinder blow by for exhaust, and as such it is not too efficient, but it does make an interesting conversation piece. As usual, I probably forgot a file or two or who knows what else, so if you have any questions, please do not hesitate to ask as I do make mistakes in plenty. Designed using Autodesk Fusion 360, sliced using Cura 2.7.0, and printed in PLA on an Ultimaker 2+ Extended and a dual extrusion Ultimaker 3 Extended.