I liked the engine (http://www.thingiverse.com/thing:25624) as printed but I wanted to make a few changes so here is my remix. It has a one-piece frame and a little tighter tolerances since today's printers are a little more accurate. If your printer is not calibrated or your settings are messed up it may take a little filing to get it to operate. The engine will run both ways now or operate as a water pump if driven by a second engine. Update: 11/3/15, Last night I noticed that the intake port is uncovered by the cylinder when the piston is on it's exhaust stroke. I have uploaded a revised cylinder that keeps the ports covered at all times. It uses far less volume of air but does not sound quite as cool. If you want the other sound use the Cylinder from the original model. This should allow people using weed sprayers to operate their engine without having to pump that handle as much. Update 11/5/15: I have tightened the tolerances on the crankshaft bushing so it fits better right off the bed.