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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 08-23-08, 05:45 PM   #1
boisePDX
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Another BB question.

Are BBs pretty much universal? Can a new bottom bracket fit into an old BB shell? or are the measurements and standards different as time goes on? (as I found out with my 1" head tube). I'm wondering what will work with my 91' Merlin. Thank you all for you help.
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Old 08-23-08, 06:04 PM   #2
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Sounds like you have a lot of questions... most of them will be answered at www.sheldonbrown.com just read his article on bottom brackets. That being said you probably just have a 68mm English threaded bb (if you can dig up a spec sheet on the bike it will probably tell you), just make sure you get the right length spindle for your crank set.
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Old 08-23-08, 06:28 PM   #3
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Thanks,

I do have a lot of questions. I have been tinkering with bikes since I was very young- usually I just take the bike into the shop and buy the part they tell me. I've been trying to learn more about the different sizes and geometry of everything. I can assemble everything if it's all right in front of me, but put me in a room by myself with a bunch of different parts and I'm pretty much lost.
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Old 08-23-08, 06:54 PM   #4
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'91 merlin, mountain bike or road bike? Probably will be an english bb. In which case a modern replacement will be easy to find. Just need to know length + shell width (68/73).
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Old 08-23-08, 07:39 PM   #5
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If you pull out the old BB, it will have the size stamped or printed on the surface... Something like 68x108.5... You'll need a replacement with the same dimensions.

In this case 68 would refer to the diameter of the BB (it would be English - 70 is Italian). 108.5 would refer to the width. Narrower or wider change your chainline and mess up how your bike shifts.

Hope this helps.
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Old 08-23-08, 08:12 PM   #6
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Originally Posted by dwr1961 View Post
If you pull out the old BB, it will have the size stamped or printed on the surface... Something like 68x108.5... You'll need a replacement with the same dimensions.

In this case 68 would refer to the diameter of the BB (it would be English - 70 is Italian). 108.5 would refer to the width. Narrower or wider change your chainline and mess up how your bike shifts.

Hope this helps.
Not quite. The 68 mm is the width of the bottom bracket shell. The diameter is 1-3/8" (aka 1.375") for an English threaded bb shell.
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Old 08-23-08, 08:40 PM   #7
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In this case 68 would refer to the diameter of the BB

Hope this helps.
lulz
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Old 08-23-08, 08:44 PM   #8
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Oh, and when you go to take out the bb, the right side (drive side) is going to be reverse threaded... clockwise to loosen.
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Old 08-23-08, 10:49 PM   #9
boisePDX
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Thanks,

All the information helps a lot. I just didn't want to buy something and then have to send it back. It's a mountain bike by the way.
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Old 08-23-08, 10:56 PM   #10
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Sometimes, after paying S&H, it's cheaper to support your LBS, instead of UPS.

http://sheldonbrown.com/harris/bottombrackets.html
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Old 08-23-08, 11:03 PM   #11
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You can measure your own spindle length so you get the right one. Just remove the cranks and then measure from end to end on the spindle in mm's. You can also measure the width of your BB shell to see what width of BB you need.

Most modern bikes use either a 68 or a 73 mm shell width. Spindle length varies from brand to brank of crankset.

If you upgrade to an ISIS or Octalink option the current 68 mm standard is a 113 mm spindle. So that's 68x113 for the size.

You can also measure the width of your BB shell on your Merlin just with a metric ruler or an inch ruler and convert. In this case what you see (from the outside) is what you get.

Not many bikes made in the US or sold by US brands use anything but English threaded BB's. Only your exotic Italian bikes may vary from this. And French offerings of course. But it's a Merlin and not a Motobecane....
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