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Old 06-14-09, 02:12 PM   #1
robo
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UN-54 BB non-drive-side cup cracked... hmm

Just noticed that the plastic non-drive side cup on my BB has cracked. I'm about 155 lbs, and there's only about 1000 miles on it. It's a Shimano UN-54.



One thing I am wondering - I installed it myself, using a torque wrench, so I'm pretty sure I didn't over torque it, but I did not chase and face the BB shell (not having the tools, and being too impatient to take it to a shop to have it done).. the frame was new, and while the threads were pretty smooth, there was definitely paint on the edge of the BB shell.

Could the uneven stress from the non-faced BB shell edge have caused the crack? Do plastic BB cups just suck?

Also, how bad would it be to keep riding on this for now?


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Old 06-14-09, 02:33 PM   #2
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They crack mainly because of overtorque. It is also conceivably possible if the threads weren't chased well enough or if the shell was *really* uneven to do it. I can see that happening.
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Old 06-14-09, 02:34 PM   #3
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Could the uneven stress from the non-faced BB shell edge have caused the crack?
Yes.

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Do plastic BB cups just suck?
Yes.
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Also, how bad would it be to keep riding on this for now?
Not bad at all.

Honestly, it's nothing to stress about. All that cup does is stabilize the non-drive end of the cartridge BB internally and give a slight sideload for the bearings. You may be able to find a salvaged metal cup that's compatible, but it'd be heavier and do no more than the plastic one.

My advice: Ride til the BB is toast, then you'll get a shiny new cup with the replacement BB.
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Old 06-14-09, 02:44 PM   #4
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Plastic cups only suck if the mechanic sucks.
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Old 06-14-09, 03:05 PM   #5
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The mechanic sucking is certainly possible

I guess my worry was the threads on the other side of the BB getting ruined if the broken cup allowed the bb to sort of wiggle..

An even worse mechanic than me (one in a bike shop, too) put in a badly fitting cup once (from the wrong BB) and that's what happened..
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Old 06-14-09, 05:47 PM   #6
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The mechanic sucking is certainly possible
I've installed many UN-54's. Cracked the left cup only once, and that was because I was clearly being an idiot.
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Old 06-14-09, 06:02 PM   #7
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I've got a cracked plastic un54 nds cup also. the mechanic was a jackass (i over-torqued it)

The instructions make no mention of a torque spec for the nds so i called the lbs i bought it from to check and he told me to torque them both to the same spec. That's when it crumbled - it was way too tight for the plastic.

i've got another one here waiting to be swapped in, got it for $5 from said lbs. Meanwhile the bike rides fine
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Old 06-14-09, 06:31 PM   #8
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Yes.


Yes.
Wrong. Wrong again.
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Old 06-14-09, 07:05 PM   #9
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Plastic BB cups
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Old 06-14-09, 07:29 PM   #10
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and he told me to torque them both to the same spec.
Which means he doesn't know what the **** he's talking about.
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Old 06-14-09, 07:59 PM   #11
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Waste of time.

Un54's are already getting resent with metal cups, stock from shimano.
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Old 06-14-09, 08:10 PM   #12
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Wrong. Wrong again.
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Waste of time.

Un54's are already getting resent with metal cups, stock from shimano.
Hunh. An aftermarket replacement and Shimano resending metal cups.

Wow, maybe plastic cups do suck.

To be honest, I've never had a problem installing them. What I have had an issue with is trying to remove them after a customer has shredded them beyond recognition...with or without the proper tool. Yes, facing has nothing to do with the non-drive cup if it doesn't bottom out against the shell. Chasing the threads (which would be part of any facing job) has everything to do with how well the threads are aligned from side to side and how easy the cup will thread in.
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Old 06-14-09, 09:42 PM   #13
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I'm fairly sure i didn't overtorque it, as I actually did destroy one by overtightening it (basically by following the instructions that came with the BB, which apparently were for a metal cup... Shimano switched the cup material but didn't update the installation instructions?)

The BB was the wrong length anyhow, so with the second BB I was careful to be much lighter. I think I got a different torque spec from somewhere online for a plastic cup. It was pretty light.


Anyhow, i guess i'll ride on it and just keep an eye on it.
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Old 06-15-09, 12:15 PM   #14
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I've cracked them, but only when being stupid. Not to imply that the OP is stupid, but we all make dumb mistakes. I tend to make them more than once, which makes me really stupid.

Your LBS can give you a new one. And yes, my newest UN54 did come with a metal rather than plastic NDS cup.
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Old 06-15-09, 01:58 PM   #15
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Almost all plastic UN-54 cups on the bikes built in our shop (and yes we use a torque wrench) develop a crack over time. BB is always faced and chased. Then again, it is no big deal. If they do not develop a creak, there is no problem.
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