Print these parts instead of the original ones if you want/have to use M4 rods&nuts and a metric OD32ID28mm plunger tube. You'll also need all of the other parts from the original, of course.
The STL files are NOT properly oriented for printing.
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Plunger head for a ID28 plunger tube (the right diameter for OD32mm PVC drain pipes you get in Europe). You'll need one 21x3.5mm (OD 28mm) O-ring.
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Modified MK18R4 parts with tighter nut slots (for M4 nuts instead of US ones. You'll want to use 2 self-locking and 2 standard nuts)
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The front and rear parts have been modified for easier tightening of the nuts
- The catch should work with about half of a ball-pen spring
The plunger tube will not sit tightly; help with some electrician's tape. It needs to be really tight (airtight, actually) in the front. Leave the back looser! Air SHOULD be able to pass there.
Please keep in mind that using strong springs might be illegal in your (European) country.