Stokestack

Hi all.

I'm overhauling my Trek 920 Singletrack.

I got a new Shimano BB-UN55 (113mm / 73mm). I had some serious trouble getting the right-hand side to screw in, but did finally get it. Don't know if it was cross-threaded or what.

Now my main question is how freely the crank shaft is supposed to spin in this thing. It's very gummy-feeling. I can't imagine the pedals spinning freely after being given a shove (I haven't attached them yet).

Is this normal? Seems like more resistance than I would expect.

Thanks for any insight.

Gavin

rhenning

Normal because of the rubber seals on the bearings. At least most of the time that is the reason. Roger

wschruba

Cartridge bearings are often somewhat drag-y when new. Without feeling it, I can't tell you if that is what you're experiencing, but that, at least, is normal.

andr0id

All normal for that BB. They are not low drag seals by any means. You won't notice it riding.

Slash5

A new cartridge BB will feel a bit sticky until the seals wear in.
How did the left side support cup feel going in? I have one bike where the threading in the BB is not aligned properly. I cannot tighten the left side cup all the way to bottom without causing the spindle to tighten up due to flexing the cartridge.

geminigeo

73mm BB sounds like an Italian thread, which has right handed threads both sides. Doubt that a Trek uses that kind of BB. My guess is you need a 68mm English BB that uses reverse threads on drive side.

andr0id

Italian is 70mm in the UN55, so I think he has the right one.

The drive side should go in all the way. No space between the shoulder and the BB shell.

The other side should only go in far enough to lock snug and support the core, Don't try to drive it in all the way to where the shoulder is flush with the BB. You usually cannot.

Wright Bros

Next time you install a cartridge BB, first screw the NDS cup in a ways to act as a cetering guide for the DS main cartridge body. Once you have the cartridge started in it's threads correctly back off the NDS cup so it won't interfere with torquing the cartridge in the DS. This makes dealing with the fine LH threads easier.

Broctoon

That's correct. 68mm is an almost universal standard for road bikes from England, USA, and Japan. 73mm is standard for mountain bikes, including the OP's Trek.

Now, the dizzying array of spindle lengths is another matter...

LesterOfPuppets

There were also tons of MTBs with 68mm shells.

Stokestack

Thanks for all that, guys.

Yes, I had a huge problem getting the RH (reverse-threaded) part to go in. I'm pretty sure I did have the other end cap screwed in at the time, but it behaved as though cross-threaded. I thought I'd ruined the bike.

After taking the LH cap out and then screwing it back in and then trying again (using exactly the logic in the above quote), I did get the RH side to screw in all the way.

So... on to the gears!