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I'm trying to replace the bottom bracket assembly on my 98' trek 820. The current one grinds and needs replacement.

The bracket that came in it is a shimano bb-ct92. I've looked online and can't find it for sale...what should i do to replace a minimum of parts and get this bracket changed out?

The dimensions are 68x116, but apparently because the front derailleur attaches to the bottom bracket, or complicates things.

Any suggestions?

Out of curiosity, how difficult/costly would it be to move to a normal style derailleur? One that would allow me to use a standard style bottom bracket?

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You could always go with a 1x setup and forget about the front mech! –  Aaron Aug 4 at 18:33
    
That appears to be a standard bottom bracket cartridge. Sheldon tells us it's square end. Any square end cartridge with the same length and offset should work. Your LBS very likely has something that will fit. –  Daniel R Hicks Aug 5 at 11:47

3 Answers 3

TL;DR - Just about any square taper bottom bracket that has a 68mm shell and roughly 116mm spindle should work fine. You should be able to find one for very little money at either your LBS or online.

As far as I can tell, the derailleur mounting to the bottom bracket doesn't make a lick of difference in selecting your bottom bracket. I've never actually used one, but from what Sheldon Brown says and the images I've found, it seems like it's just a ring that is held in place between the bottom bracket and its shell.

There are four things to worry about when replacing your bottom bracket:

  1. The threading
  2. The shell width
  3. The spindle type
  4. The spindle length.

Your bottom bracket appears to have standard English threading. This is by far the most common type of threading on the market. Just make sure that they one you buy has English threading (or, at least, doesn't list some other type of threading) and you'll be good on that front.

The shell width is the "68" in that 68x116 measurement that you mentioned. 68 and 73mm are the most common shell sizes. Many bottom brackets are sold to fit either one, some are specific to one or the other. The universal ones have a spacer that you just leave out for a 68mm shell.

Your bottom bracket appears to be a square taper bottom bracket. This means that the spindle is square and tapered. Sheldon Brown has an excellent image comparing different bottom bracket spindle types. You're lucky in that square taper bottom brackets are both common and inexpensive. There are also varying lengths of taper. Yours appears to be a JIS square taper, which is again, incredibly common and inexpensive.

Finally, you need the spindle length to match (more or less). The spindle length is the "116" in your measurement. I say it needs to more or less match because often have some wiggle room on a geared bike. Whether or not you have wiggle room, and how much you have, will depend on a lot of other factors. If you can, it's easiest to get an exact match.

For the sake of completeness, I should mention that there are also external bottom brackets. But those are a whole different ball of wax designed to work with a completely different crankset and probably not worth worrying about in your present situation.

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I think you've put together a good answer on selecting a bottom bracket but rather flippantly over looked the fitting of the E-Type FD. There are further considerations on making sure the bb doesn't fit flush in the frame and impact on chainline. I've not voted down as it isn't a wrong answer just incomplete. –  DWGKNZ Aug 5 at 4:59
    
@DWGKNZ Fair enough. I did mention that I've never used one and said that "as far as I can tell" it didn't make a difference. Feel free to edit in correct information. –  jimirings Aug 5 at 13:03

The front derailleur you are describing is called an E-Type front derailleur. This means that you have to be careful when selecting a bottom bracket to ensure that you get one that is compatible or you need to alter your chainline.

In addition to a bottom bracket having:

  • spindle length equivalent to current BB.
  • 68mm shell
  • English threaded
  • square taper

to maintain a symmetrical bottom bracket you need a bottom bracket with the spindle offset to compensate for the width of the e-type plate. These are identified with a -E mark, such as BB-UN25-E and are still available but hard to find and more a specialised product. Here is the 117mm version:

bb-un25-e

These are more difficult to find so there are options which would include:

  • Using a bottom bracket such as the BB-UN55 below which has a lip on the drive side which would secure the e-type plate (I can't comment if this would be big enough to hold your FD). This would create an asymmetrical bottom bracket that would affect your chainline but actually be within tolerabe limits without to much impact (after adjusting your FD). There may also be an issue with the cup not being fully seated against the BB on the non drive side allowing flex. This would be the minimal cost option even above finding the right BB.

BB-UN55

  • Move to a hollowtech ii external bottom bracket (would involve purchasing a new crankset) which would allow the e-type plate to be fitted with the supplied spacers. But not minimal cost.
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It appears that the BB shell is the 68mm English standard which means for very little money you can replace the bottom bracket and most likely crankset. A quick search on Jenson USA brought me this, which should come in a size that will fit your 820. If you'd like more details on the bike's specs for sizing, I'd recommend looking at it's page on bikepedia.

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