This is a drop in replacement for the stock K1 extruder to fix underextrusion and add a tensioner.

I do recommend replacing the extruder if your machine is still under warranty.  With no long term testing, I cannot speak to the longevity of this extruder.

NOTE:  I highly recommend getting root access and adjusting your extruder stepper run_current form 0.5 to 0.4.  Creality default has run current way to high causing the motor to get extremely hot.  This part must be printed in ABS/ASA or better material, PLA will deform due to heat, and some deformation was observed with ASA using the stock run current, but only on extremely long prints. 

I recommend printing the two parts with 6 walls and 20% infill at a 0.2 layer height with a 0.4mm nozzle.  

There are two designs, one that uses heatsets for the 3 holes on the body, and one that does not, but the gears must screw into the plastic to achieve proper clearances.  DO NOT OVER TIGHTEN.  The only additional part you need are one longer m3 (approximately 20mm or longer) screw with a spring and a m3 nut.

The instructions are hopefully clear enough.  If you are fitment issues with the ASA, try increasing both part 100.5% all the way around.  Fitment will depend on the tolerances of the printer used for the parts, but the tolerances are pretty tight.  

I included the step file if anyone wants to play with the design or make improvements.  This design has completely eliminated ALL of my underextrusion issues.