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. Thingiverse is going through maintenance right now, ill add the .stl and link it as soon as it comes back up.