Build guide says remove 2mm from 4mm rod and use 3mm ball bearing and drill 2mm hole in 5mm tube.So, ball must be on rod when rod slides into tube. But, 3mm ball sitting on remaining 2mm of rod is 5mm which is 1mm too large to fit into 4mm inside diameter of 5mm tube.Guide also says ball should be flush with top edge of tube. Even using a 3mm hold in the rod, 2mm of remaining rod and 3mm ball is 5mm. Inside diameter of 4mm plus 0.5mm of rod wall is 4.5mm. Ball will protrude 0.5mm.