I had some issues from the very beginning with the z motor mount. The first time i put my machine together i cracked the part where the bolts tighten up. I replaced it shortly after, but was very careful not to over tighten. Recently it actually came off of the tubes and my spindle free falled into the material. Not good…So i found an existing stronger motor mount on thingiverse and modified it. I extended the round portion down to just above the bearing plate below. Because this has a lot more bearing surface it also keeps the part more square when tightening down. Around the bolts this part is much beefier as well, and i could crank it down without cracking. This part also has the provisions to actually drill a through hole in your tubing and pass a long 6-32 bolt through all of it. There is a recessed portion for the head of the bolt and a flat for the nut on the opposing side. It turned out really great, hope someone else can use it.