With that center piece, well all of them actually, start super loose. You will need to tighten things over time but you would be surprised at how accurate this is even when it is seems too loose.
That’s the weird part the 11mm didn’t used to be standard. I shipped 13mm ones, went to a larger vendor and now I have 11mm. the 2 local hardware stores I have sell 13mm? crazy, and who knows what the rest of the world has. Tough making a machine easily sourced worldwide. I was up half the night sketching a new idea for adjustable tube size to make it easier …. doesn’t seem like it will work. Bummer