Notice The following involves use of open flames and possible temperatures exceeding 250C

Tools needed will be a good pair of plyers with rubber handles, metric socket set metric Allen key set brass wire brush and possible replacement parts such as nozzles, heater cartages and thermistors if the hot end is damaged.

There are many types of hot ends out there, although most have similar components. This guide should help for almost any hot end if you can understand the principals. There are the various MK8 hot ends the E3DV6 and its knock offs, and many others.
Hot Ends have 2 basic zones: the Melt Zone- where the filament heats to melting and is pushed out of the nozzle, and the Cooling Zone - where you are trying to rapidly cool any filament that is retracted into this zone so it doesn't melt within it, and keep the heat from building up (heat creep).

The most critical issue on assembly is that you ensure that all inner parts are tightly butting up against each other in order to avoid any material seeping out of a connection point and either oozing out of the hot end and ruining the whole assembly, or just creating a mass that will then jam the flow.

YOUR HOT END IS MOST LIKELY MADE OF THE FOLLOWING BASIC COMPONENTS (FROM THE FILAMENT FEED TO THE NOZZLE):

PTFE tube or Bowden Tube: (unless direct drive)- leads filament into hot end and keeps it running low friction, Teflon is PTFE - same non stick stuff used on pans!)

Coupling: this is a standard press fit locking mechanism to hold your PTFE tube in place, You need to make sure that it grips the tube well so if you push and pull on tube it doesn't move.

examples of types of couplers

examples of retention and locking clips

FROM HERE WE MOVE INTO THE COOLING ZONE, WHICH IS MADE OF:

Heat Sink: this is the metal finned area who's goal is to pull heat away from the filament as quickly as possible, and it's normally hooked up to the hot end fan which keep air blowing across the fins for efficient cooling.

Heat Break / Heat Throat: this is the metal threaded tube which connects the cooling zone to the Melt Zone. Sometimes the Heat Break allows the PTFE tube to pass right down inside it up to the Nozzle, other times ( as in all metal hot end) the PTFE tube butts right up against the heat break)

There should be gap here, where the Heat break is exposed.

THEN WE MOVE INTO THE MELT ZONE WHICH IS MADE OF:

The other end of the Heat Break feeds into the Heater Block.

Heater Block: an aluminum or copper block of metal which accumulates the heat and heats the filament to melt point

Heater Cartridge: a cylindrical cartridge (with two thick electrical leads) which creates the heat and transmits it to the Heater Block.

Thermistor: a thermometer (with two thin electrical leads) which reads the temperature of the heat block and send it back to the machine

Nozzle: screwed into the heat block and must but up to the Heat Break. Usually made of Brass, but may find in hardened steel, or stainless, the diameter hole is what determines the thickness of the printed filament.

DISMANTLE YOUR HOT END FROM PRINTER:

1. First remove previous filament and flush with cleaning filament if you can.

2. Heat Nozzle to temperature of last filament setting.

3. Hold Heater Block with large pliers and unscrew Nozzle with wrench when hot. (be careful not to use bare hands)

4. Completely cool down Hot End.

5. Disassemble Hot end from machine. (you may find it to be difficult to remove the Bowden Tube from the Coupling and if so, unscrew the Coupling with tube inside. You can cut it away or free it up after. Bring the Hot End to a work bench and take it all apart: unscrew Coupling, heat sink, Heat Throat, Heater Block, Heater, Thermistor, Nozzle. Note: be careful not to damage Thermistor (wires and tip are thin) when you unscrew and remove from Heater Block.

6. Inspect each part, make sure there is no residual plastic, check the Coupling, check the Heat Sink inner throat, make sure there is no plastic melted down at bottom of where Bowden Tube rests, check the Heat Throat, check the nozzle.

   If there is plastic, try scraping out with small picks or tweezers, if unsuccessful, put in a vice and use a heat gun or butane torch to melt it out, use a thin nail to push through and remove melted plastic, use Q-tips to clean if they fit.

   If your Nozzle is too blocked, throw it away and buy a new one.

Make sure each part is clean before putting back together.

REASSEMBLY, THE ORDER IS VERY IMPORTANT HERE.

Once every part is clean follow these steps:

1. Screw the Nozzle into Heater Block, hand tighten, then back off 1/4 turn (this leaves room for tightening when heated).

2. Screw end of Heat Throat into Heater Block until it hits against the Nozzle tightly. Make sure there is contact between the two, tighten the Heat Throat against the Nozzle as much as possible without damaging the parts.

3. Screw Heat Throat into the Heat Sink, make sure it goes all the way in and tightens down to the Heat Sink. Note there should be a 2-3mmm gap between the Heater Block and the Heat Sink.

   Screw on Coupling.

4. Insert Heater Cartridge and Thermistor and screw down, be careful not to over-tighten the screw that holds the Thermistor wires, you don't want to cut the jacket or short them out.
   note for ease of removal and maintenance, I put male-female connectors on all of my electric leads so I can easily swap out, remove, clean.

5. Mount Hot End Assembly to your machine. at this point I like to drop a drop of canola oil into the hot end from above to help to season the metal parts, it doesn't hurt the print, and when you first extrude some filament extrude a bit extra to remove excess oil.

6. Push Bowden Tube into Coupler, keep Coupler top pushed down, and force the Bowden Tube as far down as possible, make sure it bottoms out. Then while pushing down pull up on Coupler's top ring to have it bite into the tube and keep it in position. If possible add a coupler retainer.

7. Connect fans.

8. Heat up to max temp that you typically use 220° for PLA and 250° for PETG for example.

9. With a pliers hold Heater Block in place to not allow to move, and with a socket wrench tighten the Nozzle in place, do not over tighten as you can break off Nozzle threads in Heater Block.

10. Adjust temp to current filament.

11. Add a Silicon Sock to ensure that hot end is insulated and protected from any part fans!