Hey David,
the original Z-16 Z assembly with its 16 mm diameter chrome-plated, hardened steel hollow shafts has a 1004 ball screw. You get a replacement ball screw for this Z-16 with ball nut here. The heavier and newer Z-20 Z assembly with 20 mm diameter chrome-plated, hardened steel hollow shafts has a 1610 ball screw.
By the way, the steeper pitch of the new Z-20 assembly led to the “spindle drop with gravity on initial estop or power loss issue” that did not occur with the less steep ball screw pitch on Z-16 assembly, and made Onefinity offer a Z stepper with electromagnetic brake for the Elite Series.
However on the Z-16 Z assembly, you don’t need to change the ball screw and nut or the bearings when just replacing the 65 mm router mount by the optional 80 mm spindle mount.
Note that on the new Z-20 “heavy” slider, Onefinity does not offer a 80 mm spindle mount as a replacement part anymore. If you want a 80 mm mount on the Z-20, you got to order it with the 80 mm spindle mount from the beginning. If you order your Z-20 with 65 mm mount and want to upgrade to 80 mm later, you will be required to buy an entire Z-20 assembly 
.
Z-16 Original/X-50 default settings for motor 3:
travel-per-rev = 4,
min-soft-limit = -133,
max-velocity = 3,
Z-20 “Heavy Duty” default settings for motor 3:
travel-per-rev = 10
min-soft-limit -160,
max-velocity = 7
(settings are for units=metric)