BB Measures 67.4mm, can I use 68,73?
02-16-19, 02:01 PM
#1
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BB Measures 67.4mm, can I use 68,73?
I have a Bianchi Grizzly from the 80s. Trying to figure out what BB I can put on there (I know nothing about BBs).
Looks like nearly all MTN bikes have English threaded, 68,73 BBs. Which I understand to be the range of the width of the BB shell/housing on the bike.
Mine measures 67.4mm from end to end. Is that going to be a problem? Do I need to address that somehow?
Thanks!
Looks like nearly all MTN bikes have English threaded, 68,73 BBs. Which I understand to be the range of the width of the BB shell/housing on the bike.
Mine measures 67.4mm from end to end. Is that going to be a problem? Do I need to address that somehow?
Thanks!
02-16-19, 02:12 PM
#2
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It'll be no problem at all with a 68mm BB. Whether you use a cup and cone or modern cartridge BB, you'll just see an exposed thread or two on the NDS cup once you've fastened it down. 
02-16-19, 02:16 PM
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I wouldn't anticipate a problem. A millimeter is not generally significant in chainline issues, and if it is, a spacer may be all that is needed to address it.
02-16-19, 02:22 PM
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Generally road bikes have 68mm nominal width BB shells, ATB/mountain bikes have 73mm BB shells. You can use a 73mm BB in a 68mm shell but the
cranks are going to sit out farther and spacers may be needed, so it is not a good idea and complicates chain line and sometimes FD issues.
BB shells on metal frames are generally 'faced' to make the edges square with respect to the BB itself and this removal of metal may account for your
BB shell being a bit short. The facing is done by the frame maker usually or sometimes by the bike builder.
cranks are going to sit out farther and spacers may be needed, so it is not a good idea and complicates chain line and sometimes FD issues.
BB shells on metal frames are generally 'faced' to make the edges square with respect to the BB itself and this removal of metal may account for your
BB shell being a bit short. The facing is done by the frame maker usually or sometimes by the bike builder.
Last edited by sch; 02-16-19 at 02:25 PM.
02-16-19, 05:05 PM
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The other issue with a 73 intended BB in a 68 wide shell is that you will have 5mm less thread for the LH cup/retaining ring to overlap the shell with. Some BBs have a longer cup/retaining ring "length" so 5 threads less is no big deal. But some have already a fairly short insertion amount. For these 5 less threads might not be as secure as ideal. Andy
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AndrewRStewart
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02-16-19, 07:34 PM
#6
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Thanks all, that's helpful. Can anyone recommend internal vs external BB? I read that you should have the shell of the BB shell professionally faced if you use external since the bearings need to be well-aligned (which I would not want to do).
