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Choose large diameter knobs so they are easy to "spin" up and down the threaded rod.

I put lock nuts (regular nuts) on the other side of the tubes. Once the wing nuts were close, I spin the lock nuts up to the tubes and then tighten the wing nuts against them.

The jig calibration is very finicky. Make sure the jig is tight against the squaring plate and clamped down as tight as possible under the head. Remove and reclamp until the calibration falls within...