1) Print, Purchase and Prepare the parts.

As mentioned in the introduction, you will need to purchase 4 "AS568" #219 o-rings (1 5/16" I.D., 1 9/16" O.D, 1/8" section) for the tires and at least 1 "punch balloon" (I found mine at a local party supply store, but they also are available on line) for the "fuel tank".

I printed my parts on an Ultimaker 2+ Extended and an Ultimaker 3 Extended using .1mm vertical resolution and 100% infill for "Cylinder.stl", "Rod Balloon.stl", "Rod Balloon Coupler", "Rod Balloon Hook", "Journal Crankshaft 4mm 1.stl", "Journal Crankshaft 4mm 2.stl", "Journal Crankshaft 10mm 1.stl", "Journal Crankshaft 10mm 2.stl", "Gear Journal Crankshaft 4mm 2.stl" and "Bushing Journal Crankshaft 10mm 1.stl", 20% infill for the remaining parts, and no supports.

This is a precision print and build and as such requires a clean and level build plate (to eliminate warping) and trimming, filing and/or sanding (to remove edge oozing). 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", the cad output of Autodesk Fusion 360 and the photographs carefully noting the locations and positions of the various components as assembly proceeds.

2) Assemble and Test the Air Engine.

Slide "Rod Piston.stl" into "Head Cylinder.stl" noting the orientation of the hole in "Rod Piston.stl".

Press "Piston.stl" onto "Rod Piston.stl" positioning "Piston.stl" exactly 28mm from the end of "Rod Piston.stl". This is the cylinder head assembly.

Slide "Valve.stl" into "Cylinder Center Top Port.stl" noting the orientation of the valve holes. This is the cylinder assembly.

Press the cylinder head assembly onto the cylinder assembly. This is the air engine assembly.

At this point, as shown in the video that follows, you should be able to easily and smoothly move the valve and piston rod in and out of the cylinder. If not, disassemble the air engine assembly and lightly file until they do.

3) Assemble and Test the Crankshaft.

Slide "Journal Crankshaft 10mm 2.stl" into the hole in "Arm Piston.stl" noting the orientation of "Arm Piston.stl".

Press "Journal Crankshaft 10mm 1.stl" onto the octagonal pin on "Journal Crankshaft 10mm 2.stl" noting the orientation of each.

Slide "Journal Crankshaft 4mm 2.stl" into the hole in "Arm Valve.stl" noting the orientation of "Arm Valve.stl".

Press "Journal Crankshaft 4mm 1.stl" onto the octagonal pin on "Journal Crankshaft 4mm 2.stl" noting the orientation of each.

Slide "Journal Crankshaft 4mm 1.stl" into the hole in "Chassis Front.stl".

Press "Journal Crankshaft 10mm 2.stl" onto the octagonal pin on "Journal Crankshaft 4mm1.stl" noting the orientation of the two journals. This is the crankshaft assembly.

Press the air engine assembly into "Chassis Left.stl".

Press the crankshaft assembly onto "Chassis Left.stl", noting the orientation of each.

Press "Chassis Right.stl" first into the crankshaft assembly, then onto the air engine assembly.

Slide "Bushing Journal Crankshaft 10mm 1.stl" into the crankshaft hole in "Chassis Left.stl" then press into "Journal Crankshaft 10mm 1.stl".

Slide "Gear Journal Crankshaft 4mm 2.stl" into the crankshaft hole in "Chassis Right.stl" then press into "Journal Crankshaft 4mm 2.stl". This is the chassis assembly.

At this point, as shown in the video that follows, the crankshaft should rotate freely with no drag or catching. If it does drag or catch, carefully examine the entire crankshaft making sure it is "true" (meaning all journal surfaces are parallel to each other and the chassis surfaces, and no journal, bushing or gear rotates out of true).

4) Connect the Air Engine to the Crankshaft, Then Add the Front Wheels.

Using "Pin Arm Piston.stl", attach "Arm Piston.stl" to "Rod Piston.stl". Once attached, rotate the crankshaft to ensure no drag or catching occurs. If it does drag or catch, the air engine piston chain (includes the parts "Rod Piston.stl", "Head Cylinder.stl", "Piston.stl" and "Cylinder.stl") needs to be carefully filed until no drag or catch occurs.

Using "Pin Arm Valve.stl", attach "Arm Valve.stl" to "Valve.stl". Once attached, rotate the crankshaft to ensure no drag or catching occurs. If it does drag or catch, the air engine valve chain (includes the parts "Valve.stl", "Head Cylinder.stl" and "Cylinder.stl") needs to be carefully filed until no drag or catch occurs.

Place o-rings onto "Wheel Left Front.stl" and "Wheel Right Front.stl", then slide "Wheel Right Front.stl" into the front axle holes in the chassis assembly from the right (gear) side of the chassis assembly. Press "Wheel Left Front.stl" onto the octagonal pin on "Wheel Right Front.stl". The wheels should rotate freely with no drag or catching. If they do drag or catch, check both "Gear Journal Crankshaft 4mm 2.stl" and the gear on "Wheel Right Front.stl" for build plate ooze or extraneous filament.

With no drag or catch occurring, you should be able to blow into the top port of the air engine, give the front wheels a nudge, and the air engine, crankshaft and front wheels should all move with ease.

5) Final Assembly.

Place o-rings onto "Wheel Left Rear.stl" and "Wheel Right Rear.stl", then slide "Wheel Right Rear.stl" into the rear axle holes in the chassis assembly from the right side of the chassis assembly.

Press "Wheel Left Rear.stl" onto the octagonal pin on "Wheel Right Rear.stl". The wheels should rotate freely with no drag or catching.

Press "Adapter Balloon.stl" into the top air engine port.

Press one "Rod Balloon Coupler.stl" onto one end of a "Rod Balloon.stl".

Press the second "Rod Balloon Coupler.stl" onto the remaining end of "Rod Balloon.stl".

Press the two remaining "Rod Balloon.stl" into the remaining ends of the two "Rod Balloon Coupler.stl".

Press "Rod Balloon Hook.stl" onto one of the remaining ends of the balloon rod assembly.

Press the balloon rod assembly into the small square hole in "Chassis Front.stl".

Hook the punch balloon onto the balloon rod assembly hook.

Place the balloon open end onto the balloon adapter.

To run the engine, 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!

Congratulations, you're done!

Hope you like it!