I’m curious as to why the Z axis is set up to move 8mm per revolution. If we had a single-start thread instead of a 4-start thread, we’d have more available torque and also the Z axis wouldn’t lower when the machine shut down. Obviously it would have to spin four times as fast, but I can’t see a scenario where that would be a major problem…?
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But it’s sloooooooooooooooow… I had my Z at 2000mm/min in the end because I did a lot of inlays. I would not want to do it with 500mm/min. 
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Stepper motors lose torque quickly when you spin them fast.
Then your Z moves would be painfully slow unless you spin the steppers at a speed where they’re losing torque.
It’s an option to change leadscrew pitch but not a good trade for most.
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Thankyou both 
Personally tried 2-start thread leadscrews during the lr4 beta, was a PITA to assemble, partly because of confined access within YZ parts (they have a lot of features densely packed and optimized for rigidity/size/assembly/cost/etc…). Ultimately abandoned 2 starts and switched to regular yellow brick road 4-start leadscrews, and lived happily ever after.
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