<p>I printed a G1/8 thread in PLA where I can connect a compressor to have a small pressure container. Even though all measurements are correct and it screws in fine, it still leaks air.</p>
<p>Is there a way to make 3D prints airtight?</p>

Yes.
The issue is, that there are small gaps between the layers. But you can coat the print in an airtight material. While epoxy and similar materials work very well, they are somewhat too viscous and take a long time to cure.
My special recipe for coating PLA prints with a fast-curing airtight thin layer is:
Dissolve 1 g Paraloid B-72 in 20 ml acetone.
Dip or otherwise evenly coat the print and dry at room temperature for 10-20 minutes.