I actually had the same problem. The ACME leadscrew was the right diameter to fit in the bearing, but when I slid on the Z carriage the lead screw was about 1mm offset towards the gantry side. If the delrin nut was near the bottom of it’s travel, then I could fit the rod in the bearing, but as I brought the nut up, it would bind. Seems like some dimension was off. I ended up putting the Z-axis motor plate on a mill and extending the mounting holes for the makerslide an additional 1mm and then everything fit well.