Things to try: get a standard nut from the hardware store and check the 'slop' with that:

• If there is a lot of play it suggests a poor quality thread on the new axis you bought. I would either return the z-axis to SD or replace the thread with a good quality one from a hardware store (there are a number of posts on here detailing how to do that)

• If there is noticeably less slop with this standard nut then it suggests that the nut included with the new aluminum bed is poorly threaded. If this is the case, either return the unit to SD or see if you can replace the supplied nut with the new one you used for testing (I don't have my new alu bed and can't see from the photos on the SD website how the nut is attached to the bed). If replacing the nut is an option you should try a few from the hardware store and pick the one that gives you the most acceptable play/slop/smoothness.

Another option is to print one of the many antibacklash solutions on here.

One other thing: have you determined if your z-axis rods are parallel and true? Does the bed bind more at the top or bottom? You might have some issue there that is preventing smooth motion.

New aluminium bed fitted with new 5-1/6-18 z axis from SD store

having lots of issues with the slop in the threads and had to place extra weight to keep bed moving down correctly...

Things i have checked, Z axis rods free from corrosion and not bent, greased correctly

Z - axis thread is greased and ran die nut down no debris or burs

what would be the recommended fix for the new aluminium bed with this set-up ?

thnx