Newbie mistakes galore… On a freshly assembled printer decided to - no, not print something, I am far from that, but at least extrude some plastic, just to see how it functions. Only PLA in house (a couple of shipping mistakes left me without ABS for my initial tests, such a pity). Took 3 mm PLA, stuck it in - worked for a while, to my enjoyment, I fiddled a bit with the extruder calibration and then… the whole thing got stuck, no, STUCK. Quick googling (always done afterwards, otherwise it’s not fun!) shows the PEEK thermobarrier shoulda been severely cooled to prevent PLA from expanding in glass phaze, which is what happened to me. Just look at it - photo shows top of PEEK, where the PLA has even overflown a bit of the retaining screw…
Now the question is - how do I get all this PLA out of the PEEK thermobarrier? For the moment I am re-running the heater at 190 degrees C, in hope that after a while the PEEK will heat up evenly enough to make the stupid plastic a bit less solid. But it does not seem to happen - what else? Oven, heat it all in hope to catch it before the PEEK overheats? Drill bit, removing PTFE liner et al?
I am sure I am missing a trick here…
aaaand as is usually the case, the matter solved itself right after - oven at 200C and a wooden stick were all it took. Back to experimenting!
You can also run your hot end at ABS temperatures and push the plastic through with the same wooden stick, but the oven sounds like a more effective solution
I only count 4 flutes in that body. Meaning this is likely one of those knockoff J-heads. Don’t use clones: http://jheadnozzle.blogspot.com/2013/11/is-j-head-real.html
@ThantiK 4 flutes indeed, but I learned the difference only earlier today when I stumbled on that same post you mention. But then, this particular clone is not that bad - inner PTFE present, PTFE ring between heater block and PEEK present, even the inner nut (sorry for my ignorance of the correct term) that as I understand serves to keep the PTFE tube in place - even that is present. So, it’s not that bad, even though, of course, it’s far from the real thing.
I had suspected that a jam was caused by inadequate cooling of my j-head so I put an always-on fan on it. Months later something went wrong mid-print and the fan stopped. Strangely enough, some other print quality issues I had went away mid-print, so I left the fan disconnected since, and haven’t had a jam yet.
I think the real j-heads, with the right ambient temps, etc. can operate at PLA temps with passive cooling. I could be wrong, but so far in my testing the results have been positive.
…and if it acts up I’ll have an excuse to try an E3D 
@Eric_Moy
the problem was that PLA melted all the way up to the entrance of the PEEK tube - and no matter how I tried I could not get it hot enough to clean it all. An oven heats it all up much more evenly.
@Jason_Gullickson it seems to be affected by so many interrelated parameters that it makes my head spin. I am sure that with the right extrusion speed and heater temperature, using the original j-head with longer body and the right kind of material would be able to perform flawlessly without any active cooling 
You could try insulating wrap around the heating block. Helps my jheads avoid this type of jam on PLA.
Go figure, literally 30 minutes after I posted that my non-actively-cooled j-head jammed
(to be fair, my j-head is a bit of a frankenstien at the moment, it’s using a heater and thermistor from a junk QU-BD I had in the parts bin…)
Trying again with a fan, and making room in the budget for an upgrade…
@Jelle_Boomstra , because swapping hot ends for different types of plastic is painful and time consuming. The E3D handles everything you throw at it. The J-head does too, up to a certain temperature. Using different hot ends designed for different plastics is naive IMHO. We’re at the point where we’re going to possibly be using 4, maybe even 8 plastics at once and mixing them in ways that are yet untested. (Intermeshing ABS with PET, or HIPLA with PLA, etc)
@ThantiK yours was about the last comment that convinced me to go and shell out the electronic cash - along with making my friend do the same Knew all along E3D was great, even tried to check out local machine shops with the drawings Sanjay so kindly provides (terrible luck that was, bill estimate was about 4 times the cost of the real thing, that’s Moscow prices for you). Thought I will make do with these J-heads - nah, got myself the right one in the end. Thanks!
@Igor_Larine , I have no allegiances. I simply have a lot of experience helping people and have learned over time that certain things are just not worth the time and hassle of trying to build yourself. One of those things is the hot end. A quality hot end will save you months of hassle every time, well worth the money you spend on it.