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 Post subject: 68mm bb in a 73mm shell
PostPosted: Fri Dec 08, 2017 5:59 am 
Retro Guru

Joined: Sun Oct 26, 2014 8:34 am
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So I swapped a bb and crank between two bikes and everything seems to be working perfectly but afterwards I realised the new bike (a diamondback) has a 73mm shell.
My question is- does it matter?
Kes

P.s. Its a un71 bb


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PostPosted: Fri Dec 08, 2017 7:46 am 
King of the DuckBoard
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Location: Trimble 26er
no. As its working then all must be fine. could be issues if you change cranks? How is the chain line looking?


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 Post subject: Re:
PostPosted: Fri Dec 08, 2017 10:19 am 
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The chainline is perfect despite it being a 68mm bb

Thanks
Kes


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PostPosted: Fri Dec 08, 2017 10:26 am 
retrobike rider / Gold Trader
retrobike rider / Gold Trader

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your driveside crank will be 2.5mm further out than it would otherwise be


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 Post subject: Re:
PostPosted: Fri Dec 08, 2017 10:28 am 
retrobike rider
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It should knock the chainline in a bit, but then of course crank and what it was set at before would be needed ( if same setup then it'll be out by 2.5mm to before, so at a 50mm chainline and not the previous 47.5)

Only other thing is over time it may start to wobble as the NDS will not be clamped down fully* and the crank will be inwards 2.5mm to before.
Which might not be a problem (if say a 110 was used and not a 107)


* Though with a UN72 the clamping cup should be able to go in under the shell.


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 Post subject: Re:
PostPosted: Fri Dec 08, 2017 11:42 am 
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Joined: Sun Oct 26, 2014 8:34 am
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Thanks for your replies
Yeah I meant un72 not un71!
Anyway everything seems solid and the shifting is perfect front and rear so fingers crossed it'll be OK
Kes


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PostPosted: Mon Dec 11, 2017 8:50 am 
King of the Skip Monkeys
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Just be careful that the main part of the BB isnt floating. The end of the cartridge body will be 4 - 5mm away from its cup.


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PostPosted: Mon Dec 11, 2017 9:22 am 
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cce wrote:
your driveside crank will be 2.5mm further out than it would otherwise be

Not necessarily, probably not at all.
I assume the OP is using a shimano sq taper sealed unit. The fixed side (right hand) screws all the way to it's stop (the BB shell) and will be like it should regardless of what width it was designed for.
The left side will need to screw well inside the BB shell and so the LH crank will be nearer the LH chainstay.

My Rockhopper has run like this for about 10 years no problem


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PostPosted: Mon Dec 11, 2017 11:01 am 
retrobike rider / Gold Trader
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pigman wrote:
cce wrote:
your driveside crank will be 2.5mm further out than it would otherwise be

Not necessarily, probably not at all.
I assume the OP is using a shimano sq taper sealed unit. The fixed side (right hand) screws all the way to it's stop (the BB shell) and will be like it should regardless of what width it was designed for.
The left side will need to screw well inside the BB shell and so the LH crank will be nearer the LH chainstay.

My Rockhopper has run like this for about 10 years no problem


Essentially what this post says is that theres no difference between a 68 and 73 mm shell Shimano bb....

The fixed cups (which control the whole position of the bb) on them are slightly different to cater for the extra metal on the shell. It's just you.dont notice because it isn't a huge difference


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PostPosted: Mon Dec 11, 2017 12:55 pm 
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Location: Sheffield, top city
cce wrote:
Essentially what this post says is that theres no difference between a 68 and 73 mm shell Shimano bb....
if that's what you read, then sorry, my bad explanation. There is certainly a difference.

Quote:
The fixed cups (which control the whole position of the bb) on them are slightly different to cater for the extra metal on the shell. It's just you.dont notice because it isn't a huge difference

yeah, I think that explains what I wrote badly.

I think the driving factor on whether the OP is ok, is whether he can screw the adjustable cup far enough into the BB shell to hold the left side and stop it rocking. It is likely the whole cup will be immersed into the shell, so needs to have threads cut deep into the shell.


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