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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 02-25-06, 11:21 AM   #1
Criag Harris
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I have a old Trek 930 Single Track which is about 15 yrs old. I have rebuilt the bottom bracket cassette several times. I now need to replace it. It is a Shimano BB-CS20-68x122.5 with a left side c.w. thread. All the Shamano BB cassettes I have seem on the internet are right side thread. Which model should I be looking for? I live in Northern India so there is no bike store just down the road. I have to order parts from Canada and wait for what I hope is the right part. Can anyone help me
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Old 02-25-06, 12:18 PM   #2
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All Treks take "English" threaded bottom brackets. Yours will take any Shimano square taper bb with the correct spindle length so just match the length in your current bb. I recommend a BB-UN73 or BB-UN53, whichever you can get in the correct 122.5 mm length.

BTW, it doesn't matter which side the cartridge is on and which the plain collar is.
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Old 02-27-11, 03:31 PM   #3
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anyone ever(correctly) answer this one?

I recently acquired an old 80's Falcon Manta MTB with an unusual tracer triple crankset ,the Shimano CS20 122.5mm square taper BB was shot so I tried replacing it with a fairly good condition UN51 122.5mm BB,But the crank ended up contacting the frame & sat much further in on this identical length BB.So either the tapers on the CS20 are fatter or it very much does make a difference which end the plain or adjustable collar is on.

I really like the crankset but am really confused about what to do about the BB

Any ideas anyone ?
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Old 02-27-11, 03:44 PM   #4
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Hmm..actually the side of the spindle that the chainring arm attaches to is noticeably fatter with less taper on it,never seen one like it before. think I'm off to my LBS to see if they've got an old 127mm that I can try(will this place the non-drive side too far out?)

appreciate any help
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Old 02-27-11, 07:25 PM   #5
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Looks like an older Octalink system

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Originally Posted by Criag Harris View Post
I have a old Trek 930 Single Track which is about 15 yrs old. I have rebuilt the bottom bracket cassette several times. I now need to replace it. It is a Shimano BB-CS20-68x122.5 with a left side c.w. thread. All the Shamano BB cassettes I have seem on the internet are right side thread. Which model should I be looking for? I live in Northern India so there is no bike store just down the road. I have to order parts from Canada and wait for what I hope is the right part. Can anyone help me
Hi Craig _ any references I`ve found to a Shimano BB CS20 indicate its an older Octalink bottom bracket and since its a road bike - probably a V1 version. There is definately a drive side and a non-drive side.

V2 versions are more common and were introduced later and found mostly on mtb bikes.

Have actually located a few shops in England that have that actual model in stock. It lists for less than 15 pounds sterling. Is there a reason you can`t order from there?
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Old 02-28-11, 08:11 AM   #6
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Thanks for the input but its a square taper(y'know just like I said),I am aware there is an octolink version but this is not the one I am after nor is Craig as I believe he has the same one as me with the fixed & adjustable cups the other way round to normal(with the adjustable on the drive side), weird I know.

This Shimano BB seems to be rebuildable as there is a narrow nut & threaded section on the non-drive side(another reason I beleive mine & Craig's are the same BB)

Like I said the drive side tapers are noticeably fatter than the non -drive side(again very weird) so I am going to try a 127mm standard Shimano BB with my Tracer crank & will post again if this works.

I can see from the original posts that I am reaching back through time somewhat but,Craig did you ever sort out your BB issue?
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Old 02-28-11, 11:44 AM   #7
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Since you actually have one in your hands

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Originally Posted by nomonkeybizness View Post
Thanks for the input but its a square taper(y'know just like I said),I am aware there is an octolink version but this is not the one I am after nor is Craig as I believe he has the same one as me with the fixed & adjustable cups the other way round to normal(with the adjustable on the drive side), weird I know.

This Shimano BB seems to be rebuildable as there is a narrow nut & threaded section on the non-drive side(another reason I beleive mine & Craig's are the same BB)

Like I said the drive side tapers are noticeably fatter than the non -drive side(again very weird) so I am going to try a 127mm standard Shimano BB with my Tracer crank & will post again if this works.

I can see from the original posts that I am reaching back through time somewhat but,Craig did you ever sort out your BB issue?
I think that`s a pretty good reference.

I dug up my info from J E Cycles in the UK and they not only had that model listed in the Octalink section - they even had a photo of an octalink BB posted as a reference. The problem with all that is that it could all be due to a bad link.

Yeah - I expect the OP probably got his own issues sorted out. Have you looked at UN51 / 71 / 91 BB to see if the square tapers are compatible with what you have?
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Old 03-07-11, 08:13 AM   #8
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Hi Burton,

you are quite correct in the fact that there is a octolink BB with the name BB-CS20,I am aware of this(although I think this BB had a 73mm shell),there is also is an older BB with the identical name(BB-CS20) which is square taper,this BB has been discontinued for a long time now,mine was found on a 1980's MTB,(although I don't know if this was original)

This one had the fixed & adjustable cups the other way round to normal with the adjustable on the drive side(unlike the later octo' which had them the normal way round),I dont know why Shimano used the same name/code for 2 totally different BB's(perhaps as they had stopped making the former some time before)

Anyway the solution was simple enough,although the CS20 was 122.5mm in spindle length,because of its fatter drive side tapers the crank arm sat further out than it would on a standard BB of the same measurement,so I invested in a 127mm UN53 BB & low & behold a perfect fit & decent chainline,problem solved.
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Old 03-07-11, 08:18 AM   #9
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sorry for the repetition in that post ,forgot what I had already written ;-)
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