REQUIRES 4 PRINTED BEARINGS THAT ARE NOT INCLUDED IN THE STL LIST BELOW PLEASE PRINT 4 HERRINGBONE PLANETARY GEAR BY PRUSA RESEARCH FOUND IN MK3S SAMPLE OBJECTS ON PRINTABLES (or included on the sd that came with prusa printers)

Print tips are included in each model relating to perimeters, support, or infill settings. In general, all parts were printed at a .2mm layer height with a .4mm nozzle. Most parts were printed with 3 perimeters, some require at least 3 to print as intended. Most print orientations are obvious, but I included some in the notes.

For fasteners, I would suggest printing all similar height bolts (or all of them) at once to give plenty of time to cool layers while printing the threads. Two perimeters worked somewhat better for the screws, and adding a brim that connected them all together on the bed also helped, along with slowing down print speed.

I wet sanded any moving parts (pistons, cylinder walls, slides, gear mounts, exhaust “valves”, etc.) with fine finishing sandpaper and that greatly helped with friction, I would also recommend using some sort of lubricant for moving parts - I personally use old silicone lubricant made for rubik's cubes that I had sitting around. Check individual parts after sanding to ensure they move with little friction before putting the motor together.

Build tips: In order, assumed that parts are already sanded and ready to go in

Tools needed: 5mm allen key, 10mm wrench

Press the bearing retainers into the crankcase (one A and B per each gap).

Assemble the crankshaft and bearings (shown in picture, 180 degrees between the crank arms), then place the assembly into the crankcase – push all shafts into slots as far as they will go and make sure elbow pins are fully seated in the arms. Light taps with a hammer was helpful. During assembly, pay careful attention when assembling the outShaft part of the crank; the shaft is difficult to remove from the crank arm or flywheel once seated and the notch to align the gear means you can't slide everything on from one end. The gear slides on from the flywheel side and seats against the notch with a bearing added before the flywheel. 

Bolt the bearing retainer C on top of each bearing using crankBearingBolts.

Slide the wrist pin into the piston and thru the connecting rod, tighten with a 5mm allen key.

Hold the connecting rod against the elbow pin on the crank, slide the conRodCap over the end to lock it to the crank, then use the conRodBolt and conRodPin to fasten the cap to the rod.

Slide the pistons into the bottom of the cylinders and bolt the cylinders down with the regular bolts - start with the middle two on each side and that will be enough to hold it together for now.

Add the intermediate gear to the bottom mount on the geartrain, then add the two slideGears to the upper mounts. Mount them so the top of the slot (marked with a small line) on the gear on the right is facing directly towards the small air hole on the right, and the left gear with the slot directly away from the air hole on the left (use the mounting holes for the slide block to line up). Make marks on the teeth of the gears where they mesh to keep track of this position. 

Add the slide block and place the slides into the block and the slot on the gears. Place the cover on top and fasten with a bolt and a spacer. DO NOT tighten too much, as the bolt will have to come out again to add the nozzle later. Press the geartrainRetainer onto the gear mounts and ensure they sit around the slides and are not pushing them (this is a good spot to sand as well). Sandwich the geartrain between the geartrainMount and the geartrainSupport and fasten together with geartrainBolts.

TIMING: This can be changed a bit with testing and rotating the geartrain independently from the crank to change when air goes into the cylinder. The instructions will follow how I have mine set up. – Turn the crankshaft until cylinder 1 (closest to flywheel) is at top dead center. For the motor to rotate COUNTERCLOCKWISE, spin the right hand slide gear COUNTERCLOCKWISE from the originally marked position until the air hole behind the slide is just barely visible. This will open the cylinder to let the air in right as the piston starts to move downwards. I would assume there is some degree of advance (counterclockwise rotation of the right side gear) that would make more power/spin faster but I have not tested this.

Mount the geartrain to the cylinders/crankcase - the intermediate gear on the bottom should slide into the gap in the crankcase; ensure that the timing is not knocked off while putting it in. Fasten with the geartrainMountBolts on the third hole from the flywheel and the regular bolt on the hole closest. The second set of holes is unused.

Place the exhaust valves into the head plates and set the manifold on top. Hold them together and push them into the top of the cylinders – the geartrain will flex enough to slip the manifold over it, if you are worried you can losen the bolts some to allow more play. Fasten the head down with regular bolts and a spacer. 

Undo the bolt on the slide block, add the nozzle, and fasten again. Add the remaining bolts to the cylinders to bolt it to the crankcase, and the motor is ready! Start with one of the cylinders just past top dead center and it should be ready for air, it is a good idea to wear safety glasses and durable gloves when dealing with high pressure air and plastic as a precaution.