Nozzle Cleaning

After twice backing up plastic into the Bowden tube, and discovering I could no longer push plastic through a hot nozzle by hand, I finally had to admit that the extruder nozzle was completely blocked.
So, here's what I did to unclog it.
First, remove the brass tube and nozzle from the extruder head. This means dissembling the head, removing the PEEK insulator and then using a wrench to unscrew the nozzle and tube from the aluminum block containing the heater.
I tried to separate the 6mm brass threaded rod from the nozzle, but stopped after mangling the threads in the vice - brass is really soft and no matter what you wrap the threads with, it isn't soft enough to not damage the threads and yet stiff enough to hold it fast.

The nozzle is finally clear.

Next put the tube and nozzle combo into the oven and turn the oven to the hottest it will go. In the winter here it might have gotten to 250°C. I used a bent paper clip arrangement with one end stuck up the tube to hold it so that the nozzle is pointing up, but in hindsight I think that a better way would be to not have anything stuck up the tube at all. After 15 minutes or so at that temperature, this leaves a small puddle of caramelized plastic on the protective foil on the pan under it.
Then use a bath of acetone and a Pasteur pipette to bathe the interior and exterior. You need to mechanically clear the blockage. There are very few things that will poke into a 0.5mm hole. A pin or needle is too large after the first millimeter.I finally settled on the ink plunger from an old Staedtler Mars mechanical pen I had. If you take it apart, the 0.5mm pen has a fairly stiff wire about 15mm long that one can stick into the nozzle hole.
You can't imagine the relief when the wire can finally fully enter the nozzle.
Keep cleaning with the wire from the nozzle side and a long pin from the tube side until the nozzle is clear.
You can switch to a bath of vinegar and salt (home made brass cleaner) to clean up the rest until the black crud stops coming out. Acetone is too volatile and won't dissolve the burnt stuff very well. I think if I was in North America, I might try oven cleaner.
Finally, run a 6mm x 1mm metric die over the threads to clean them up after the mauling they got.

By the way, to get plastic out of a Bowden tube is not so easy either. You have to squirt acetone into the tube end, bend the tube back and forth (as much as you can) and pick at the blockage with a pin and poke at it from behind with the 3mm plastic stock, until the wad is removed.