karldub

Hi,

I have a 80-90's Concorde race frame with a vintage Dura Ace Crankset. The other day, my crank arm started to unscrew itself from the BB and ultimately came off completely. The crank arm has a crack in the arm which might be causing it to unscrew. However, I'm not sure the BB is broken or if it's supposed to look like this. It looks like one part of the bolt, the "thread stem" is merged/a part of the bottom bracket and then theres just a small nut to screw back.

I have tried to screw the crank arm back with help of the nut, but after a couple of miles it goes back to keeps unscrewing itself.




Is this bottom bracket also broken or is this some niche type of crank set? Or can I extract the "threaded" stick that comes out of the BB, or is this an integrated part of the BB?

jdawginsc 

You can remove all of the bottom bracket to degrease and repack. You can check for cracks, but a picture of the crank arm might help.

Its an older Bb axle, where the attachment is male, rather than the female of most modern BBs. It is likely that either it was not torqued enough and the taper hole expanded or the crack makes proper torque impossible.

hope that helped!

PhilFo 

Did the bottom bracket cup unscrew from the frame or just the cracked crankarm?
That "threaded stick" is the bottom bracket spindle, and also is a standard one, the way it was constructed on most square taper spindles for years. Many inexpensive bikes can still be found with this type of spindle. If your bottom bracket cup is still firmly fixed in the frame, then it sounds like what you need is a new crankarm. I ask about the cup itself since yours in the photo is Italian thread and the drive side can work themselves loose because of the right hand thread on both drive and non-drive side.
You could probably best be served by completely removing the bottom bracket, measuring the spindle and installing a cartridge unit. By the look of that spindle, your bottom bracket needs the maintenance at the very least and may need replacement if there is any serious pitting of the races.

SurferRosa

None of us would use a cracked arm, except maybe to hang on a garage wall.

RustyJames 

Wrong taper on the BB for the crank arms?

Kai Winters

It's just an old style bottom bracket. It is easily replaced but you do need three tools to do it. One is the spanner for the center ring...the one with the holes in it. The second is to remove the lock ring...the outer ring with the notches in it. The last is a hammer which is very likely needed to 'crack' the seal of the lock ring. It's not at all hard to do with the right tools.
If you want to keep the bike I suggest removing the bottom bracket and replace it with a modern sealed unit with a tapered, square or diamond, bottom bracket. Make sure you get the correct bottom bracket threading.
Replace the crankset...no way should you use a cracked arm and anyway it won't work with a more modern...at least compared to the original...bottom bracket. Then off you go.

merziac

steelbikeguy

My impression is that the nut keeps loosening because that crack is in the crank. The crack is probably expanding when the nut is tightened, preventing the threaded stud from developing enough tension to keep the nut in place.
This is just another reason that the cracked crank needs to be replaced.

As a side note... I'm familiar with those sorts of bottom brackets axles (i.e. ones with threaded studs instead of being threaded internally) coming with low-end Sugino cranks. I wouldn't associate them Dura-Ace, although I could be wrong. The cheap Sugino ones wore out somewhat quickly, so it's worth pulling yours out and checking.... although you'll probably need a different bottom bracket assembly for whatever new crank you install.

Steve in Peoria

Steel1

As noted above, please inspect the crank arm for the crack that you mentioned above. A Cracked crank arm is seriously dangerous. It is not fun when they finally choose to snap in two, usually when you stand to apply extra force. I would NOT use any crank arm with a crack.

Kontact

Common and undamaged BB. Broken crank. Throw the crank away.

steelbikeguy

just out of curiosity.... can you give us more details about the crack that you found in the crank arm?
I'm visualizing it as starting at the four sided hole for the bottom bracket axle, probably starting at one of the corners. This is why I imagine that it is impossible to properly tighten up the nut on the axle.

I get the impression that others might be imagining that it is located somewhere along the length of the arm, such that if it fails, the pedal will suddenly break loose of the bike. This is certainly a safety risk, but wouldn't impact the ability to attach the crank arm to the bottom bracket axle.

Steve in Peoria

Kontact

I had assumed the crank is cracked where it goes on the BB spindle.

karldub

Thanks for the helpful input! See pictures of the crack in the crankarm below...

mpetry912 

with a crack like that the arm will never stay tight. You can find a replacement on ebay or other source.

/markp

Trakhak

I hope this isn't one of those "I paid little or nothing for the bike, so I'm determined to put no money into it" situations.

That crank would break in the not-too-distant future if you continued to try to ride with it. Possibly with nasty consequences for you.

A local bike shop should have new left cranks in stock that are the correct taper (to match the four-sided tapered bottom bracket spindle) and the correct length. Probably around 30 dollars or so. (Going on ancient pricing knowledge, but shouldn't be much more.)

mpetry912 

putting "Left crank arm" into ebay I get many options for under 20 bucks.

but what do I know ?

/markp

Trakhak

By the way, to clear up potential confusion:

Concorde was a brand of high-end Italian racing bike frames available in the late '80's, and Dura-Ace is, of course, Dura-Ace, but this bike seems likely to be one of the low-end Concord(e)s that were sold in department stores and the like in the '70's and early '80's. And the Dura-Ace cranks would likely be from the first generation, when they were mid-level at best.

Could be wrong, but nothing in the photos provided so far suggests a high-quality bike, to me, anyway.

merziac

You are lucky this did not already pile drive you.

Any time you see a crack like that its time to scrap it, NOW!

macstuff 

Here's your chance to put in a modern JIS cartridge style BB.
Anywhere from $25 to $125 depending on quality.
That old style Japanese BB axle is not JIS but the Dura Ace Crank set is.
The difference in the tapers and set depths is most likely what cracked the arm when it was last installed.
With a new JIS cartridge, arm and proper torque, you'll be good-to-go.
The new cartridge will come with the proper crank arm bolts.

macstuff 

... Is the reason.
It will happen again with a replacement JIS arm. It needs a new JIS BB and a replacement arm.
That crank set did not come on the bike, the BB most likely did.

jethin

No possible problems there — totally fine to ride.

(Sorry, just yankin’ yer crank stick.)

merziac

Disagree on the mismatch, I have logged 1000's of miles on cranks and spindles that may or may not have been matched.

Never a crack or failure, ever, period.

Its more likely this was far overtorqued at some or many points in time, or flawed to begin with, the overbuild and wiggle room on these is well above most any unforseen kludge.

macstuff 

You'll have to forgive me when I say that's a Dunning-Kreuger answer.
The inverse of that is, I have seen it at least a dozen times at the shop over time and I've done it myself with a female axle version of the same type BB and the same 7400 series arm.

The issue is that the arm has a taper and is deeper than the axle square is long. When the serrated axle nut is tightened down, it forces the taper over the shoulder of the square on the axle, thus spreading the taper hole. It doesn't happen on the drive side because there is more meat on that side spreading out the deformity.
The mismatch is real and is easily and cheaply addressed.

The OPs BB shown is typical of a 80s/90s box store bike. It is not a JIS square taper BB, it is a shallow square. Those sometimes even originally had steel cranks attached to them. A Dura Ace crank of any ilk is JIS and has no business on that BB.
Might as well force an octalink arm on there. Maybe a cottered arm then drive in a set screw...

Advising people to continue the mismatch is irresponsible. Especially if it could end in injury... And it could if the arm chunks off while riding.
It's like saying it's ok for everyone to run stoplights because you do it all the time and you never got in a wreck.
Pfft.

PhilFo 

If that's the original spindle, then it is a JIS taper as that's a SR SVX bottom bracket. The crankarm was probably not adequately tightened (thin wall socket needed) then ridden for however long until the arm split.

macstuff 

Nope. ALL the nuts that go with these BOTL BBs have serrated shoulders that act as a lock. If tightened even snug, they don't back off by themselves.
​​​​​​Bicycles are the beginning and bottom rung of motive mechanics. Not a speculative science. Easy and Finite.
Next will come the metallurgical OCD guy and on and on...
Aaaand I'm finished.
You-all do what you do.
👍