Repeating horizontal bands are a reason to inspect the Z motion, but their spacing is a clue rather than a verdict. Record the pattern before loosening hardware.
Measure the repetition
Measure across several bands and divide by the number of gaps. Compare the result with the screw's specified lead: the distance travelled in one revolution. Pitch and lead are not interchangeable on a multi-start screw.
For a specified four-start screw with 2 mm pitch, lead is 4 × 2 = 8 mm. That calculation does not establish that every screw called “T8” has the same lead, or that an 8 mm band proves the screw is bent.
Separate observations from causes
Print the same simple test with a known working material profile. Record whether the bands repeat at the same heights, whether they appear on all faces and whether the pattern changes with speed or temperature settings.
Check the printer's maintenance instructions for the Z assembly. Inspect accessible fasteners, guides and screws for contamination, damage or looseness using the prescribed power state and support procedure. Do not disconnect a load-bearing screw and let a gantry fall as a binding test.
Repair against the machine's specification
Use the maker's instructions for alignment, lubrication and replacement. A coupler gap, nut preload or top bearing arrangement from a different printer is not a universal upgrade.
Prusa's MK4/S and MK3.9/S maintenance guidance is an example of why instructions must be scoped to a model: it distinguishes lubricated smooth rods from Z screws and trapezoid nuts that must not be lubricated. Do not assume every screw and nut should be cleaned with solvent and greased.
If a screw is visibly damaged or measurements are outside the manufacturer's tolerance, seek the approved replacement procedure. This guide does not prescribe bending a lead screw in a vise or invent a universal runout limit.
Confirm the result
Change one justified item, then repeat the original test and photograph it under the same lighting. Keep the failed and successful samples together with the settings.
If the symptom persists, send the printer support team the model, firmware, photos, measured spacing and work already performed. A complete diagnostic record is more useful than a succession of unverified mechanical modifications.
