This is an M12 screw and matching nut design with an extra feature, it has a 4mm cavity hole. The cavity hole is intended to accept a 4mm PTFE tube. The PTFE inner diameter is 2mm. The PTFE tube can be used to pass 3D 1.75mm filament through it.

You may ask the question, is this PTFE and printed hollow bolt approach as good as a PC4 pneumatic fitting? The answer is likely, not quite, but keep in mind, the fitting is not working under pressure, hence air leakage will be low. So you can expect good results.

Depending on the slicer setting, the inner hole may not be exactly 4mm. I had to resize the inner hole slightly to be 4mm with the aid of a 4mm drill bit.

The printed M12 screws and nuts needed a little attention to improve their screw threads. You can simply use a metal M12 nut and bolt for this purpose; even better if you have an M12 tap and die to finish them off.

These 3D printed nuts and bolts should only be finger tightened. Care! Over tightening of the nut will sheer the screw from its head.

Here is my 3D design. My design may match your needs, too.

My print is made with eSUN PLA+ 1.75mm Black filament, with 0.2mm layer height and 15% infill on a Bambu X1 Carbon printer. Print supports are not required.

I hope that you can find this print useful, too.

Remember to leave me some feedback or even a tip to keep me enthused to make more of these drawings.