Changing z axis leadscrew

Hello Folks,

I have built an MPCNC primo with a Chinese 2.2 kw spindle, but the weight of the spindle causes the z axis to drop when the power is switched off.
To counter this, have changed the z axis leadscrew from a 2 start thread to a single start thread, changing the mm/rev from 4 to 2.
My problem now is how to change the z axis steps/mm? Have recompiled Marlin with DEFAULT_AXIS_STEPS_PER_UNIT changed from {200,200,800,200} to {200,200,1600,200} but the z axis is still moving only 5mm instead of 10mm when moved from the LCD. The settings on the LCD are showing z axis steps/mm set to 1600.
Is there something I have missed?
Thank you for any help anyone can offer.
David H.

First, typically I see 4-start and 1-start lead screws mentioned on this forum, so your numbers may be off. As for your problem, one possibility is that you’ve set the steps per mm using g-code at some point. If so, any changes in the firmware for values set and saved by g-code are ignored. If this is the root of your problem, you can either use an M92 g-code to set your steps-per-unit, or you can do a factory reset (M502) to clear all parameters that have been set in g-code and therefore the values set in firmware will be used.

Edit: if the LCD is driven by Marlin and showing 1600, then my possible problem is not your issue. If the firmware values are being read and it has the same movement as before, the only thing I can think of is that the micro-stepping was somehow changed. You might also test the movement with g-code as well as the LCD.

Thank you Robert.
Your first thought was exactly right. My original leadscrew was 4 start, not 2 start. Should have taken a better look.
Setting the step/mm to 3200 fixed it.
Much appreciated.