Designed this as a birthday gift for a mechanic. Simple model to demonstrate the way a flat engine works. Crankshaft might be a bit fragile when turning. Don't tighten the screws too much, since there might be some friction between the pistons and the cylinders. Let me know how the Connections work out for you and I might upload a customizable model to adjust tolerances between parts.

I printed all the moving parts with black filament and the housing with Velleman's Neutral, which is somewhat transparent. This way you can see the movement of the engine.