There may be another reason, if it's the same like for the MGN12 linear guides: with MGN12, there are Hiwin linear guides with 2.381 mm diameter balls, and there are clones with 2.5 mm diameter balls. One cannot put the Hiwin ball carriages on the clone guides, and not other round also (much too loose or much too stiff). The guides have different shapes also. Maybe the guide or carriage in the original thread was not an original Hiwin one. I tend to buy guide + carriages together to be sure they match.