Extra parts needed:

• M3x8 Bolts (x15)
• M3 Nuts (x15)
• Nitrile Gloves (thumbs from M sized gloves worked the best) or Silicon (worse) or PET (not tested) ones

Optional parts:

• M3x12 or longer bolts (2x)(for the gears)
• M3 Nuts (x2)(for the gears)
• 6x4mm Silicon Pipe (can be found in aquarium shops)
• M5x10mm Washers (x2) (instead of printed ones)
• 105ZZ Bearings (x2) (noticed no difference in performance)

Printed well with 0.6 nozzle at 0.2 layer height. Some parts (Cylider_Back_Lid and Valve_Lid_6mm) need supports. After printing cut away the brims from Cylinder_Ring parts.Parts with X_ in their names are optional.

Assembly:

• Cut off a glove finger tip (thumbs from M sized glove worked well)
• Pull it onto a Cylinder_Ring (Use Helper_Finger to leave enough height)
• Connect a Piston to it with two Washers
• Clamp the Cylinder_Ring between Cylinder_Plate and and Cylinder_Lid
• Clip them with Helper_Clamp so they won't fall apart at once
• Bolt them together to Plate_Top
• Repeat for the other 2 cylinders
• Insert th Crank with 2 Washers
• Fix it in place with Frame_Bottom
• Bolt all the parts together
• Put the Valve_Inner in place
• Bolt the Valve_Lid into place
• The engine should be running!

Troubleshooting:

• Jams on a certain cylinder - glove finger may be too small, and the cylinder washer is catching on it when passing the cylinder ring.
• Isn't rotated freely by hand - gears bolted too tightly to frame (should have a bit of play), some gear teeth didn't print very well, or the whole gear printed with elephant foot and runs too tight with another gear.
• Rotated freely by hand, but struggling under air pressure - the valve may have too much friction against the frame surface when pinned down to it by pressure (may help sanding the inner valve surface a bit).