RE: Different Z lead screw travel between MK4S upgrade screws and Core One kit — 2 mm mismatch
IMO, it's certainly desirable that the leadscrew pitches match, but a total dimensional error should be the only effect. The error is distributed over the travel distance, so there's never a problem layer-to-layer. It's not going to crash or shift, at least I don't think so. The system also has enough flex or slop so that a couple mm difference shouldn't cause significant lateral tension, friction or binding of any sort. That's easy enough to test by manually rotating one screw to see how far you can go before it binds. These are rolled leadscrews, not ground, and I suspect they have a wider than might be desired tolerance, especially batch to batch. Being a fast multi-start screw, it doesn't take much error in the screw to cause a position error of a mm or so over a long distance. What would really worry me is if there were a single rotation "drunken screw" problem, because that would cause a local issue between layers.
RE:
Hi jürgen,
I respectfully disagree that this only affects dimensional accuracy. In a system where the bed moves away from the nozzle, a pitch discrepancy creates a dynamic skew.
I did not mean to say (and did not say, I think) that it is just a constant scale factor. "Dimensional error" is a broader term in my understanding. I fully agree, the effect will be a linear distortion of the shape.