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Was it not that UN-7x was like xt/xtr lever with the metal n/d cup, un-5x was the deore/lx type spec one and the un-2x is fairly recent and much lower budget (so presumably poorer quality addition to the lineup?
I thought the un-7x series stopped being produced when square taper went out of favour so we are now left to choose btween the 5x and 2x series.
M_V-
To be quite honest, I don't know.
I know this:
UN-7x = both cup are metal and removable.
UN-5x = NDS cup are metal and removable.
UN-2x = NDS cup are plastic and removable.I cannot comment on the cartridge bearing as I don't know if it's any difference thorough the series.
However with bicycle components, they often get tinker down as technology advanced, such as the latest Nexus 8 speed hubs whose internal is basically the older generation Alfine 8 speed.
This is why I assumed that the UN-5x replaced the UN-7x.
edscoble
Fair enough, I had another look and the UN-72 is one of the old design where both cup are not fixed to the body.
I think the UN55 is the replacement of the UN-72 (the UN-26 have a plastic non driveside cup), the newer Shimano threaded bottom bracket nowadays have an intergrated cup on the driveside.