Hi everyone - I want to replace the bottom bracket on my Raleigh Twenty. I'm interested in going the Shimano UN72 with Phil Wood retaining rings route. I know lots of other folks have done this before and I was wondering if someone knew the proper bb size. Currently Nashbar has this bb in sizes 70x115, 70x113, 70x107 - but the bb is also available in sizes 68x107 and 68x113.
I think you need a 73x113 or 73x115. The bottom bracket shell is unusually wide in the 20 so you need a longer bearing to be sure the rings will clamp down before you run out of threads. The axle length you need depends on what kind of gearing you use but a long axle (at least 110) should work.
BTW: don't let them sell you a UN73 instead of a UN72. Many places consider them interchangable but that's not true for the Twenty because one end of the bearing is threaded on the UN73.
when you get it you'll need to remove the black alloy collars from the cartridge - some people have had no trouble doing this by simply putting the thing in a vice and knocking out the cartridge with a wooden or rubber mallet (or a real hammer with some carpet or similar deadening material between the contact areas.
In my case though - the collars were glued on with this white gak and in the end I had to saw one (carefully) off with a junior hacksaw. It took two cuts on opposite sides and some persuasion with a stone chisel to finally get it off. But it was worth it. I think the oft used phrase is 'buttery smooth'