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Old 08-02-08, 02:12 PM   #1
datlas
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BB issues

I have a clunky bottom bracket and have a few concerns.

First, the bike is 22 years old, with Dura-Ace 7400 components. I estimate the total mileage on it to be about 20-25K.

The BB was serviced/repacked one year ago.

About 3 weeks ago I noticed a clunk from the bb area, which gradually became stronger and louder. More noticeable under load....I took it to my LBS (not my first choice but close to work) and they said the BB was loose and needed an adjustment...they did the work, and the bike was fine....for about 12 rides (approx 300 miles)....now the clunk is back.

I brought it back to the LBS and the wrench there spent 10 seconds pushing on the pedal in the 6:00 position and said the BB is not loose, and if the clunk is back then I need a new bottom bracket.

My two questions/issues:

1. Based on the above story do I really need a new BB or should I get a second opinion from another mechanic? I suspect I may need a new BB but the re-evaluation seemed perfunctory.

2. Given my "vintage" components, should I seek out a NOS shimano D-A or 600 BB from the 80's? Or just get an off-the-shelf square taper BB from the LBS of whatever brand they have?? I suspect it doesn't matter, but I don't know much about bottom brackets. Obviously I can't get a modern D-A or ultegra one since they have gone to octalink, and I need a square taper to fit my crankset.

3. Given the scenario above, am I risking anything by doing some more rides on the bike? I dont suppose there will be any catastrophic failure but I dont want the bike to fall apart in the middle of a ride.

Any comments/suggestions appreciated!!

Doug
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Old 08-02-08, 02:41 PM   #2
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If there is a problem, it WILL get worse if you ride it.
It may still be at a stage where it can be saved with a couple $ worth of new ball bearings and fresh grease. It may also be at a stage where the spindle & cups are damaged.
You won't know without taking it apart and inspecting. If the cups & spindle are toast, get a new cartridge unit. Even if you can locate new cups, the price isn't really cost feasible.
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Old 08-02-08, 02:51 PM   #3
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Old 08-02-08, 02:58 PM   #4
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Learn to feel for the play yourself. Could most likely be that the crankarm bolts are backing out and your crankarms' taper are gouged out and no longer seats on the BB spindle correctly.
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Old 08-02-08, 03:06 PM   #5
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There are two types of 7400 cranks/BB; low profile type that take a cartridge BB with 103mm spindle (arms are rounded on this model) and the earlier type with the loose ball type BB (not sure how long but longer than 103) - this crank has squared off straight arms.

Sounds like you need a new BB. Since you haven't learned about components yourself you have little choice but to trust your LBS. Ask for their recommendation on a new cartridge type BB and be happy.
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Old 08-02-08, 03:10 PM   #6
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http://www.sheldonbrown.com/creaks.html
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Old 08-02-08, 03:47 PM   #7
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There are two types of 7400 cranks/BB; low profile type that take a cartridge BB with 103mm spindle (arms are rounded on this model) and the earlier type with the loose ball type BB (not sure how long but longer than 103) - this crank has squared off straight arms.
The late-80s pre-LP style FC-7400 used a 109mm spindle. This was the shortest available at the time. There was only a 2mm gap between the crankarms and the BB-cup. The low-profile arms that came out after that had thinner cross-section and a shorter seating area between the BB-spindle and crankarm. Some people tried fitting the earlier arms to the newer BB, but it didn't work, the arms would bump up against the BB cups.

In the OP's case, it's hard to tell whether it's the BB that actually gets loose or it's the fit of the crankarms on the spindle. Ultimate test is when you feel play, remove the arms and move spindle up and down by hand. The loose-ball shimano BBs had rubber contact seals that hides small amounts of play. The trick to getting these adjusted properly was to install the drive-side crankarm on first before making bearing-preload adjustments on the adjustable cup. Then you can feel the play better.

The other trick was you had to tighten the adjustable up slightly tighter than the spot of smooth-rotation without play. Then when you tightened the lockring, it pulls the cup out slightly and the adjustment is then perfect.

Last edited by DannoXYZ; 08-02-08 at 03:59 PM.
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Old 08-02-08, 06:04 PM   #8
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Thanks for the suggestions. I don't have the tools or expertise to adjust or take the BB apart (I have a crankarm extractor and can remove cranks, that's as close as I get). My components are all from 1986 so it sounds like my BB is the loose bearing type. I know the axle is a square taper.

Sounds like I have to trust the LBS and bite the bullet and get a new BB.
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Old 08-02-08, 06:14 PM   #9
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Well, if you have a crank-extractor, take off the cranks and feel the BB yourself. Simple enough. I suspect that your crankarms are toast.
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Old 08-03-08, 12:41 AM   #10
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I've used a large curved jaw vise grip pliers for a cup/lock ring tool and a couple small screw drivers for the adjustable cup.
If it was recently "serviced", you shouldn't have to worry about things being "frozen".
EDIT-
IF the current BB is "toast", you haven't really lost anything by trying to fix it yourself! It MIGHT be a simple repair?
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Old 08-06-08, 07:00 AM   #11
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Bump: for appreciation, clarification, and an interesting update!

First, thanks for all the suggestions. I do appreciate them. I also understand that is difficult to diagnose a problem online without seeing the bike.

That being said, I also wanted to clarify: I cannot get the BB to clunk or click when I am off the bike. Only while riding, and even then only under load, most notable when mashing up hills at 60 rpm. When spinning the clunk is more subtle and sometimes not even present.

When I pull and turn the cranks and BB off the bike, everything is smooth. The crankarms are tight on the BB spindle, and the chainring bolts are also tight.

Also, I am somewhat reluctant to take the BB apart, since I ride every day (commute and recreation) and don't have a beater/backup, I am concerned that I might put the bike out of commission...lame exuse, but that is it.

Now for an interesting update....I went out for a ride this a.m., it was raining a bit, the roads were quite wet, and as the drivetrain (and I) got wet, the clunking seemed to go away. Within 30 minutes, the clunking was totally gone. Not sure if this is a temporary fix (which is what I suspect), but I am wondering if the collective wisdom here has any additional insight given this latest turn of events!

In the end I will probably have the LBS replace the BB anyway since it is 22 years old, but now I am more confused than ever!

Doug
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Old 08-06-08, 08:34 PM   #12
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Sometimes the faces of the bottom bracket lug aren't square. The cups will screw into the threads fine, but then when the lock ring is cranked down, the lug distorts and the bearing balls are not under uniform tension as they roll in their races, causing the clunk when pressed hard as in sprinting and climbing.

One solution is to have the frame faced by a shop that knows what it's doing. The other is to simply buy a cartridge bottom bracket that's compatible with your cranks.
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Old 08-07-08, 12:08 PM   #13
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Can't really tell if you need a new BB or not, but if you do, get a cartridge one and not a "vintage" cup and loose bearings. While loose bearing BBs can last for a long time (my 22-year-old 7400 with > 50K miles is still going strong), you'll likely have fewer problems with a good cartridge and they don't need as much maintenance.

The Dura-Ace 7400 cranks require a 68x112mm axle (or 70x113mm for 70mm BB shells). The 103mm axle is for the 7410 (8-speed from early 90's). 68x112mm is not a common cartidge BB size but 68x113mm is and should work.

Last edited by Gonzo Bob; 08-07-08 at 12:16 PM.
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Old 08-07-08, 12:32 PM   #14
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Can't really tell if you need a new BB or not, but if you do, get a cartridge one and not a "vintage" cup and loose bearings. While loose bearing BBs can last for a long time (my 22-year-old 7400 with > 50K miles is still going strong), you'll likely have fewer problems with a good cartridge and they don't need as much maintenance.

The Dura-Ace 7400 cranks require a 68x112mm axle (or 70x113mm for 70mm BB shells). The 103mm axle is for the 7410 (8-speed from early 90's). 68x112mm is not a common cartidge BB size but 68x113mm is and should work.
Bob, thanks for the reply. I looked up my BB on sheldon's site, and told the LBS that I needed a 68x112mm one....they insisted on measuring it and said it is 68x113....I figure one mm in the axle won't cause a crisis...and I figure even if the "real" problem is not the BB, it is still a good idea to get a new cartridge one.

Doug
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