1) Print and Prepare the Parts. I have attached a PDF file containing the list of printed parts which includes the part names, part counts, print settings and colors I used. You will need to have a 1/4 thread compressor adapter and compressor to operate this model. Prior to assembly, test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding may be required. Carefully file all edges that contacted the build plate to make absolutely sure that all build plate "ooze" is removed and that all edges are smooth. I used small jewelers files and plenty of patience to perform this step. 2) Assemble the Block. Assemble a piston by placing the smaller end of one "Connecting Rod.stl" into one "Piston.stl", and secure in place with one "Piston Pin.stl". Repeat this process for the remaining 3 pistons. Test fit the piston assemblies in "Block.stl" to make sure they slide smoothly with no binding or grabbing. Position on "Journal Main.stl" as shown in "Block.stl". Slide one piston assembly into the cylinder from the top of the block as shown. Center the large hole in the connecting rod over the hole in the journal main, then press one "Journal Rod.stl" into the hole in journal main as shown. Test to make sure the assembly rotates with ease. Repeat this process for the remaining pistons and journals carefully noting the orientation of each journal assembly as shown. Press "Cap Crankshaft.stl" onto the first main journal as shown. Press "Gear Crankshaft.stl" onto the last main journal as shown. Press "Head.stl" onto the block assembly as shown. Slide "Rotor.stl" into the head as shown. With the rotor aligned as shown and the first journal rotated such that the piston is at the top of its stroke, press "Gear Rotor.stl" onto the rotor as shown. The position is important for successful operation of the engine. Thread a compressor fitting into "Adapter Compressor.stl" as shown. Press the adapter assembly into the head as shown. 3) Test the Air Engine. I've included a slow motion video of the engine in operation using an air compressor with about 8psi of air pressure. That's how I made my Four Cylinder Air Engine. Hope you like it!