First layer height problem after changing nozzle I have a problem with my first layer height. But lets start at the beginning:

I own a Tevo Tarantula Dual extruder printer with a endstop switch for Z axis. 
No Auto bed level. 0.4mm nozzle.
Everything worked as it should. I home extruder with G28 command and level the bed using a piece of paper or a thickness gauge 0.1mm. In the slicer I set Initial layer height 0.2mm for layer height 0.1mm. The fisrt Layer looked perfect and sticked to the bed fine.
a few days ago it stopped in the middle of a print, nozzle clogged.
No problem i changed the nozzle to a spare one i had. It is not the first time i took it apart.

Here starts my problem:
After everything was build together again i home the extruder with G28, level the bed and start a print with the same settings i usualy used to print.
But either the first layer didn't stick to the bed at all or looks terrible.
i tried to level the bed again and also tried to get it to work with playing with the settings. But with no satisfactorily results.

Here is is the part my Question starts:
I noticed that the first layer is actually not at the same Z=0 if i use G28 to home and the Z0 it shows when i print a part.
For example: 
Set G28 Level bed with 0.1mm thickness gauge.
Then start printing with layer height 0.1mm and initial layer height setting 0.2mm

The result should be 0.1+0.1+0.2 =0.4 real layer height (if i also count the 0.1 layer heigth(not sure if this is correct)) the printer shows here a Z of 0.2
But if i meassure it, it is around 0.7mm thats way to much.



I am confused why this is the case after i changed the nozzle. Considering i leveled the bed the exact same way i did before and used the same slicer.
The new nozzle is a little bit shorter but i made sure both nozzle of the dual extruder have the same height. 
I do not understand why this happens. Is there a way to reset the Z? 
I tried the M851 code but as far as i understand it, it i used for Auto-Bed leveling. It didn't work anyway, says unknown command.

I could work around the problem by adjust the endstop until it somehow works, but then if i want to level the bed and use the G28 code the nozzle would crash into the bed and that is not a satisfactory solution.
Would it suffice to set a z-offset in the firmware? I don't have much experience on that area.


  ;FLAVOR:Marlin
  ;TIME:737
  ;Filament used: 0.351177m
  ;Layer height: 0.1
  ;Generated with Cura_SteamEngine 3.2.0
  M190 S47
  M104 S197
  M109 S197
  M82 ;absolute extrusion mode
  G21 ;metric values
  G90 ;absolute positioning
  M82 ;set extruder to absolute mode
  M107 ;start with the fan off
  G28 X0 Y0 ;move X/Y to min endstops
  G28 Z0 ;move Z to min endstops
  G1 Z15.0 F9000 ;move the platform down 15mm
  G92 E0 ;zero the extruded length
  G1 F200 E3 ;extrude 3mm of feed stock
  G92 E0 ;zero the extruded length again
  G1 F9000
  ;Put printing message on LCD screen
  M117 Printing...
  ;LAYER_COUNT:9
  ;LAYER:0
  M107
  M204 S2200
  G0 F3000 X75.185 Y64.596 Z0.2
  M204 S1750
  ;TYPE:SKIRT 
  G1 F1650 X76.267 Y63.316 E0.06132
this is a common case in my TT....
Please check the y-carriage wheels - on the 2040 and 2020 profile.
Mostly you shall find a little wobble on one of the ends, that introduces instability in leveling, so you could level it, home it and another level is needed. 
In my case, I decided to upgrade for dual-z drive - please see this

Another interesting Z upgrade:
option two

We can observe z-wobble by watching the x-axis on the left side (no lead screw), it will have a little delay in response when moving up/down comparing to the other side.