Joined: Aug 2010
Posts: 12,688
Likes: 2,094
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
I am not sure what a UN51 is, I have a UN55 on my expedition bike, nothing wrong with it.
With 15k miles on your old one, might as well put on a new one when you are at it since they are about $20 or so. Since I do not know what a UN51 is, I assume that the UN55 uses the same tool to install in the frame, but you would need to verify that.