This is meant to be a one-way check valve between two parts of a water tube. Feel free to copy and customize your own diameters.

After trying several thingiverse check valves, I wasn't satisfied. So I tried to come up with one of my own :)

Main part:

• 18mm inner diameter
• contains 16mm standard marble (you can buy it in toy stores)
• bottom part is 'curved' to allow marble to 'sit' on the bottom when blocking
• holes on sides for a 'pin' blocker

Pin blocker:

• simple stick to go through the main part and block the marble from coming out

Connector:

• 16mm outside diameter in order to fit tightly inside the main part's larger end

Note on tolerance: My Ender machine is quite old so the tolerances and quality for me came out "ok", but water was still seeping out about 1 drop every 2 seconds when I tried it. Still, quite good as far as I can print it at 0.12mm.

Probably better machines will achieve a tighter result and either completely block water or very slow drip. Good for most purposes imho.

Can even server as a 'constant drip' valve for plants, now that I think of it :P

Let me know in comments or messages what you think! Thanks!