UPDATE: I added a V2 teppet. It sits tighter into the cylinder and causes less blowby, causing a more efficient run at even lower air pressure.

This engine is inspired by Tom Stantons Compressed Air Engine.
But with some small changes. Toms engine has about 2.7ccm, mine has about 15mm capacity.
It has way more torque at higher pressures. I tested it with up to 16 bar (232psi). But of cause it’s less efficient, even if it runs on very low pressure too (about 0.2 bar or 3 psi).
I didn't like the solution to glue the cylinder head to the crankcase. So in my version they are screwed with 4 M3x10 screws. Just like the cylinder head. 
It needs a heavy flywheel to run smooth. I use a 160mm rotor from a bicycle. But you can easily make your own (for dimensions take a look into the photos if you haven`t already).

BUILD
The building procedure is just like the one of Toms engine is. Link to his video down below.
As valve I do not use a rubber seal and a BB pellet like Tom does. I found a rubber ball with 6mm diameter which works just as well (link below).
The bearing is very tight insight of the crankcase. Take a small hammer and hammer it carefully in place. Don’t be afraid of breaking anything.
Also I changed the pressfit air connector to a simpler design. It’s just a bicycle tubes AV valve. You should get one for free in your local bikeshop.
The head gasket is made of bicycle tube as well.
All parts are printed with a 0.6mm nozzle. 

What you need: 
Bearing dimensions: 16x31x10mm
8 M3x10 allen screws for cylinder and cylinder head
2 M3x15 piston and connecting rod
2 M3x20 for flywheel and tappet. You need to sand down the head of the tappet-screw (picture)
Rubber ball 6mm diameter. Mine is from an old bike pump

Links: 
Toms Video: https://www.youtube.com/watch?v=YLPY8CY5Fm4&t=730s
Rubber ball: https://www.bicycles.de/p/sks-ventilset-fuer-vx-kompressor/8146706/?gclid=CjwKCAiAo7HwBRBKEiwAvC_Q8RU4yS8geeQgu-MLbJCzaCxkI-chVI87wnEUfQEUJ0OsL8bXnklP8hoCiMkQAvD_BwE

EDIT:
Sorry for the complete mess with the pictures when you downloaded them. I don't know what the heck thingiverse is doing there.... You can download them here: https://www.dropbox.com/sh/l131ctee5q5t9vj/AABXvaJrRZYtSF09hHnYk1DGa?dl=0

Last but not least: 
If you like to support me take a look to my youtube channel. Its not related to DIY stuff, but on cool mountain bike videos. So if you like watch some bike videos while waiting for your printer ;) 
https://www.youtube.com/channel/UCpvRg6Igsevopo50N8gS-Fw