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I need to replace the entire drivetrain on my circa 2000 MTB commuter. Quality groupsets all seem to use external bearings these days. Replacing my current cartridge bottom bracket for a modern external bearing BB is my main compatibility concern. What information do I need from my frame and a prospective groupset to determine if it will fit my bike?

Off the top of my head, I guess I need to know my frame's BB shell width and thread and the chainline. What else? Where do I find the corresponding info for the new parts, particularly Shimano? Anything I need to look out for?


Update: The bike needs new a new bottom bracket, chainrings, chain, 8speed cassette and front derailleur. Replacing them individually costs almost as much as a complete modern groupset (which includes new levers, rear mech and two more gears).

Why not buy a new bike? Three reasons. One economic: the steel frame is sound, the wheels and brakes are new, and bikes of comparable quality are much more expensive than a new groupset. One philosophical: I don't like throwing things out while they still work. One emotional/irrational: I like my bike, it's been good to me, and I want to do right by it.

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    By the time you buy an entire drive train, you can often afford to replace the entire bike, or not be very far from it. – Kibbee Feb 26 '15 at 20:54
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    There are a lot of good quality square taper cranksets and corresponding cranksets on the market still (for not so much). And you won't get benefits really from a external bearing system on a commuter. – Batman Feb 26 '15 at 21:38
  • I'd have to disagree there, @Batman. My commuter gets filthy. I love my external BB when it comes time to disassemble, clean, and reassemble. It's soooo much easier than a cartridge-style BB. – jimchristie Feb 26 '15 at 22:28
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You actually guessed the only thing that you really need to watch out for: the BB shell width. And you don't even necessarily need that until you're actually putting it all together. Almost all BB shells are 68 or 73 mm. Most of the external BB's come built for a 73 mm shell and include a spacer for a 68 mm shell.

The odds are really really good that the threads and diameter coform to the British I.S.O. standard. If you have reason to think that your bike is non-standard, I'd recommend taking it to a shop and they can tell you what you need.

Unless your mountiain bike is a single speed, you don't need to worry about chainline. They're pretty standard and a derailleur adjustment will take care of any deviation from your current setup. If it is single speed, the BB isn't where you'll be looking to get that right. It's not like a cartridge-style BB where you can just get a different length spindle. Instead, you move the chain rings to the inner or outer side of the spider and/or mess with spacers in the rear wheel.

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Note that spindle length is independant of the frame, and depends only on the crankset. This makes the shimano external bearing system is easy to install since the cups work with any standard BB shell width, and the spindle is integral to the right crank of the crankset (i.e. no worries about 113/117.5/123mm etc... spindles as you would have to consider for square taper to achieve a correct chainline).

For a shimano Hollowtech II external bearing MTB setup you'll just need to know the threading (almost certainly English) of the BB shell, and the width (68mm or 73mm).

If your width is 68mm then you'll use the spacers included with the external bearing BB assembly when screwing the cups into the frame, and if 73mm then you don't use the spacers.

You then install the Hollowtech II crankset by inserting it through the BB, attach the left crank arm, and tighten the correct bolts to secure it.

It is as close to plug and play as you can get really. However, you will have to consider what number of speeds (7/8/9/10?) your drivetrain is and match the crankset for optimal performance. I've only ever seen 9spd and up external bearing setups.

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