The TWSBI Diamond series of fountain pens from TWSBI are all designed to be user-serviceable. Although the instructions are quite comprehensive, sometimes a little extra help is needed.
This video shows you how to remove and re-attach both the nib assembly and piston mechanism from a TWSBI Diamond 580 fountain pen.
The video also covers how to re-assemble (ie repair) the piston mechanism if something goes wrong. Skip to 2:30 for how to get the piston mechanism properly aligned. At 2:21, when the piston knob is screwed back onto the housing (part 11 in the instructions), turn it so the open end of the knob is just lined up with the point where the housing steps up in diameter. When the mechanism is properly reassembled there should be 2.5 to 3mm of space between the rubber piston seal (part 9) and the housing when the piston is fully drawn back (3:07).
Note that TWSBI Vac models have a completely different filling mechanism and this has a right-hand thread, not the left-hand thread of the pen in this video.