Hi there.

I've installed my Shimano BB-UN26 square type bottom bracket to my bike with one question.
I've applied specified torque to drive side first then attached the plastic cup of BB.
But the outer edge of plastic cup does not sit flush to the frame when screwed all the way in. 3-4 threads are visible.
Is it normal? Please answer.

Best regards to all.

You aren't trying to fit a 70 or 73mm BB into a 68mm shell, are you?

That's what I was thinking too. I'm thinking the shell width should be marked on the cartridge. that number should match your bottom bracket shell width. How does the space between your left pedal and the chainstay compare with your right pedal distance? That's not a 100% test, but it will get you in the ballpark.

I like to just start the drive side of the bottom bracket and then install the nylon non-drive side first. I can't bring myself to put 30 ft/lbs of torque on the nylon part either. Then, lastly, I torque down the drive side.

No, the shell is 68mm and the broken BB which I removed from my bike is a 68mm BB of another brand. I'll double check chainline but new BB is what Shimano specifies that to be used with my own crankarm.

I've never damaged a plastic cup by applying full specified torque. You are using a torque wrench and following Shimano's instructions (grease on threads & torque value), right?

This is normal.

Here is the sequence that is used to install:
1. Tighten the drive side cup in place.
2. Tighten the plastic cup in place to the correct torque.
3. Back off the drive side cup a little and then tighten the plastic cup a tiny bit.
4. Re-tighten the drive side cup to the correct torque spec.

You may still have a small gap between the lip on the plastic cup and the face of the bottom bracket shell...do not be bothered by this.

One other thing you can do is wrap the threads of the cups with teflon tape (plubers tape)...it helps to prevent the development of bottom bracket creaks.

-j

+1. The torque between the cup and the cartridge unit is what's most important, not that the lip seats perfectly against the bottom bracket shell.

I've noticed this too, plastic cups sometimes aren't consistent with the BB they come with. Some threads exposed isn't pretty, but no problem either.

I must have been half asleep when I wrote the above...here is the correction

1. Tighten the drive side cup in place to the correct torque spec.
2. Tighten the plastic cup in place.
3. Back off the drive side cup a little and then tighten the plastic cup a tiny bit.
4. Re-tighten the drive side cup to the correct torque spec.

The instructions, for whatever that may be worth, have a torque spec for both cups, even those included with my UN26s. The only caveat given for plastic cups is not to use grease on the threads.

The problem is that the shimano instruction sheet shows a picture of the cartridge with a cup on it which in reality is usually the metal drive side cup, and then also shows a picture of the other cup labeled as "adapter"...above the two images is states: "Adapter/Bottom Bracket Tightening Torque: 50-70NM (435-608 in-lbs)...

...this would lead one to believe shimano expects the plastic adapter to be tightened to 50-70NM which will likely result in destruction of the plastic splines where the metal install tool interfaces or cause the plastic cup to otherwise break.

The 50-70NM is the torque spec for the metal drive side cup. The issue then becomes how tight to make the plastic cup and the general consensus is tight enough that it properly supports the cartridge and won't come loose. To achieve this you need to install the metal drive side cup first to torque and then back it off a hair...then install the plastic cup until it does not thread in further and then tighten the metal cup to the proper torque spec again which will press the cartridge against the plastic cup snuggly enough that it should not back out and will properly support the BB cartridge.

Nothing greater for a newbie mechanic to destroy their part on installation while following the manufacturers vague instructions...had a friend do this in my garage as he got overzealous and did not wait for me as I stepped out to take a phone call. I came back in and found him sulking over the broken part and the install sheet that came in the package. Luckily nothing else was damaged (frame/cartridge) as I had a spare cup in my parts bin and we got the install done properly.

-j