The Extruder - Printing in Plastic: Build Your Own 3D Printer - page 245

Figure 14-5. Insert the PTFE tube into the feed cylinder.
Push the PTFE tube into the feed cylinder (pointed end first) as far as possible. You can use a sharp
blade or pen to make a small mark on the tube at the end of the feed cylinder. Pull out the tube and place
it over the feed cylinder and heater core so that the mark you made matches up to the end of the feed
cylinder. If the tapered end of the tube doesn't extend to the tip of the nozzle, you didn't push it in far
enough; try again and make another mark until you get it all the way into the nozzle.
Once the tube is inserted properly, use a sharp blade to cut the tube so that it is flush with the end of
the feed cylinder, as shown in Figure 14-6.