I printed my parts on an Ultimaker 2+ Extended and an Ultimaker 3 Extended using .1mm vertical resolution and 100% infill for "Cylinder.stl" and "Rod Balloon.stl", 50% infill for the remaining parts, and no supports. 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 a flat jewelers file and plenty of patience to perform this step. Study "Assembly.stl" carefully noting the locations and positions of the various components as assembly proceeds. Slide "Rod Piston.stl" into "Head Cylinder.stl" carefully noting the orientation of both parts. Next, press "Piston.stl" onto "Rod Piston.stl" until it is exactly 28mm from the end. If the piston is not tight on the piston rod, a small dot of thick cyanoacrylate glue can be used to hold the piston square to the piston rod and in position. This is the cylinder head assembly. Slide "Valve.stl" into "Cylinder.stl", carefully noting the orientation of "Valve.stl". The valve should easily slide in and out of the cylinder. If not, "surface sand" both sides of the valve until it does. To surface sand, I use an 8 by 10 sheet of 220 grit sandpaper laid flat on my work table, then using light pressure, rub one side of the valve for 20 strokes, then the other side for 20 strokes. Clean the valve of sanding dust, then try the fit again and repeat as necessary until the valve slides with ease in and out of the cylinder. Press the cylinder head assembly onto the cylinder aligning the valve slot with the valve. Once attached, check that both the piston rod and valve easily slide in and out with ease. This is a critical step as both the piston and valve must slide with ease. Attach "Arm Piston.stl" to the piston rod using "Pin Arm Piston.stl". Once attached, "Arm Piston.stl" should swing freely on the piston rod. Attach "Arm Valve.stl" to the valve arm using "Pin Arm Valve.stl". Once attached, "Arm Valve.stl" should swing freely on the valve arm. Slide the axle of "Journal Crankshaft 10mm 2.stl" into the hole at the end of "Arm Piston.stl". Once in place, press "Journal Crankshaft 10mm 1.stl" onto the axle of "Journal Crankshaft 10mm 2.stl" carefully noting the orientation of both. Make sure the axle of "Journal Crankshaft 10mm 2.stl" fully inserted into the hole in "Journal Crankshaft 10mm 1.stl". This is the piston journal assembly. Slide the axle of "Journal Crankshaft 4mm 2.stl" into the hole at the end of "Arm Valve.stl". Once in place, press "Journal Crankshaft 4mm 1.stl" onto the axle of "Journal Crankshaft 4mm 2.stl" carefully noting the orientation of both. Make sure the axle of "Journal Crankshaft 4mm 2.stl" fully inserted into the hole in "Journal Crankshaft 4mm 1.stl". This is the valve journal assembly. Press "Chassis Left.stl" onto the left side of the air engine assembly. Slide the axle of "Journal Crankshaft 10mm 1.stl" into the hole in "Chassis Left.stl". Note that if the hole spacing in "Chassis Left.stl" does not match the pin spacing on the air engine assembly, then the cylinder of the air engine assembly is not fully seated in the head. Press "Chassis Nose.stl" onto "Chassis Left.stl" making sure it is fully seated. Slide the axle of "Journal Crankshaft 4mm 1.stl" into the hole in "Chassis Nose.stl", then this axle into the hole in "Journal Crankshaft 10mm 2.stl" carefully noting the orientation of both journal assemblies (when the piston journal assembly is pointing straight back towards the cylinder head, the valve journal assembly must be pointing straight up). Press "Chassis Right.stl" into "Chassis Nose.stl",making sure it is fully seated. Press "Chassis Right.stl" onto the right side of the air engine assembly. Note that if the hole spacing in "Chassis Right.stl" does not match the pin spacing on the air engine assembly, then the cylinder of the air engine assembly is not fully seated in the head. At this point the crankshaft should rotate with ease and the piston and valve assemblies should slide with ease. You should feel no binding or catching as the crankshaft is rotated. Install the one o-rings onto the wheels. Press "Wheel Left Front.stl" onto the axle of "Journal Crankshaft 10mm 1.stl" making sure it is fully seated. Press the axle of "Wheel Right Front.stl" into the hole of "Journal Crankshaft 4mm 2.stl" making sure it is fully seated. With both front wheels installed, the front wheels should rotate freely with no binding or catching. Slide "Wheel Right Rear.stl" into the rear of the chassis from the right side, then press "Wheel Left Rear.stl" onto the axle of "Wheel Right Rear.stl". With both rear wheels installed, the rear wheels should rotate freely with no binding or catching. Press the 3 "Rod Balloon.stl" rods together using the 2 "Rod Balloon Coupler.stl" rod couplers. Press "Rod Balloon Coupler Hook.stl" onto one end of the balloon rod assembly. Press the remaining end of the balloon rod assembly into the small hole in "Chassis Nose.stl". Attach a punch balloon to "Adapter Balloon.stl", press the adapter into the upper port on the air engine assembly, then hook the punch balloon to the balloon rod hook. To break the vehicle in, I placed small drops of light machine oil on the moving parts (axles, journals, pins, etc.) then removed "Adapter Balloon.stl" and installed "Adapter Compressor.stl" (with a compressor quick connect) in its place, and ran the vehicle for 30 minutes at around .5psi. To run the vehicle, I use "Adapter Balloon.stl" as a mouth piece to inflate the punch balloon, after inflating I press my thumb onto the balloon end of the adapter to seal the air, then press the adapter into the air engine upper port and off it goes! Good luck, and I hope you like it!