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Old 01-12-11, 10:05 PM   #1
acyclist
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Crank arm + JB weld. Is this a bad idea?

My left crank arm, attached to a splined bottom bracket, manages to wiggle itself loose every time I ride. I've tried thread locker and tightening the bolt as far as I dared. It is now to the point that it must be tightened every 10 minutes and it's getting worse because the splines are wearing down.

I'm consider putting JB weld on the splines and on the threads of the bolt. When I eventually want to remove the crank arm I plan on using a butane torch heat the whole area and break the bond. JB weld is rated to 500 degrees F and a butane torch can reach 2500 degrees F.

Am I running a risk of permanently attaching these cheap components to my jamis sputnik?
At the moment I can't afford a new bottom bracket and cranks.

Thanks for your help.

Last edited by acyclist; 01-13-11 at 02:28 AM.
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Old 01-12-11, 10:12 PM   #2
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you would be much better off just buying a cheap used crankset
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Old 01-12-11, 10:16 PM   #3
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I wouldn't use JB Weld on any part of my bicycle, ever. You aren't going to be able to isolate the heat when you attempt to break it free.
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Old 01-12-11, 10:19 PM   #4
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thanks for the input guys, that was quick!

at what temperature would I risk damaging the steel frame? Perhaps I could choose an adhesive that degrades at a lower temperature.
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Old 01-12-11, 10:20 PM   #5
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I have a suspicion that the JB weld would break loose under the torque
of riding eventually, not in 10 minutes but at some point not usefully longer.
Crank is toast, BB almost toast. If you are in a club, check around for a
cast off crank (amazing what people squirrel away) and spring for a new
BB. FWIW L crank arms alone are pretty cheap in the older series.
Steel frame, if protected from flame splash over not likely to get hot
enough to damage it. Paint may not do as well. Hope you have some thick gloves
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Old 01-12-11, 10:22 PM   #6
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you would be much better off just buying a cheap used crankset
+1... check ebay, there are zillions
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Old 01-12-11, 10:27 PM   #7
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Sounds like threads in the spindle are stripped. However, on chance that the problem is the crank bolt, if you haven't done so already, you might try replacing the bolt. Also, what kind of cranks and BB? I thought Sputnik had a cartridge BB rather than crankset with integrated spindle. If thats the case, you could replace the BB for next to nothing; you'd just need to match spindle length so chainline with your cranks is right.
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Old 01-12-11, 10:27 PM   #8
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i would probably taking a hammer to the left crank arm. hammering it tighter onto the bottom bracket and then tightening them up again.
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Old 01-12-11, 10:57 PM   #9
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You don't want to heat the spindle end of a crank arm. This is a highly stressed area, and your JB weld solution may change a nuisance to a possible cause of injury. As someone who's broken a number of cranks and pedal spindles in his career, I can assure you that it's no fun at all.
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Old 01-12-11, 11:24 PM   #10
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i would probably taking a hammer to the left crank arm. hammering it tighter onto the bottom bracket and then tightening them up again.
Nope- not going to help. Once the splines develop some play in them no amount of force will re-form the crank into the correct shape. Besides, hammering on a crankarm will destroy the bottom bracket bearings in short order unless the other side is supported.

IMO: replace the left arm at very least. Tighten it correctly, with a socket and a torque wrench. There's no way to get it tight enough with a little 6 or 8-inch long Allen wrench.
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Old 01-12-11, 11:33 PM   #11
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It's time to spend a little money. No way out of it. bk
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Old 01-13-11, 01:43 AM   #12
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Nope- not going to help. Once the splines develop some play in them no amount of force will re-form the crank into the correct shape. Besides, hammering on a crankarm will destroy the bottom bracket bearings in short order unless the other side is supported.

IMO: replace the left arm at very least. Tighten it correctly, with a socket and a torque wrench. There's no way to get it tight enough with a little 6 or 8-inch long Allen wrench.
i didn't say it would re-form the crank. i just know from experience that a few small taps can sometimes push the crank arm on a bit farther and allow the bolt to be put in farther which can stop the crank arm from working its way loose.
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Old 01-13-11, 08:20 AM   #13
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....you could replace the BB for next to nothing; you'd just need to match spindle length so chainline with your cranks is right.
If the crank arms won't stay tight, it's almost certain the mounting holes in the aluminum arms themselves are distorted, not the splines of the steel bottom bracket spindle. Replacing the bb without replacing the crank itself will not accomplish anything.
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Old 01-13-11, 09:22 AM   #14
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Quote:
Originally Posted by thirdgenbird View Post
you would be much better off just buying a cheap used crankset
This
Quote:
Originally Posted by bikenut2011 View Post
+1... check ebay, there are zillions
This
Quote:
Originally Posted by Jeff Wills View Post
IMO: replace the left arm at very least. Tighten it correctly, with a socket and a torque wrench. There's no way to get it tight enough with a little 6 or 8-inch long Allen wrench.
or this
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Old 01-13-11, 11:17 AM   #15
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If the crank arms won't stay tight, it's almost certain the mounting holes in the aluminum arms themselves are distorted, not the splines of the steel bottom bracket spindle. Replacing the bb without replacing the crank itself will not accomplish anything.
Yes, if the crank arm is distorted, replacing BB is not going to help. However, I had situation recently where threads in BB spindle itself were damaged so bolt got loose and then crank arm got loose. Fortuneately, splines of crank did not get distorted and replacing BB for 20 bucks fixed the problem without expense of cranks. At the time, I figuired I'd try cheap option first -- replace BB, -- if after that I discovered distorted splines, then I would swallow expense of new cranks to go with new BB. Six months later, cranks are tight as day I put em on the new BB.
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Old 01-13-11, 11:29 AM   #16
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If it was mine and i had no money,I would give it a shot.Might work,might not.If it stays tight,great.If it doesn't,so what,your back to square one,big deal.

If you do try it,use regular JB,not JB QUIK.Regular JB is alot stronger and give it the full 24 hours to cure.It has to be clean enough to eat off of to give it a fighting chance.
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Old 01-21-11, 10:57 PM   #17
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I took the advice of jeff wills and found a tool to tighten it down properly. I also put a good amount of teflon tape on to the threads of the bolt. It held perfectly tight for over a week now so things are looking good. If it comes loose again, I won't be using jb weld since the consensus seems to be against it.

Thanks for your help.
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Old 01-22-11, 07:23 AM   #18
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Originally Posted by acyclist View Post
My left crank arm, attached to a splined bottom bracket, manages to wiggle itself loose every time I ride. I've tried thread locker and tightening the bolt as far as I dared. It is now to the point that it must be tightened every 10 minutes and it's getting worse because the splines are wearing down.

I'm consider putting JB weld on the splines and on the threads of the bolt. When I eventually want to remove the crank arm I plan on using a butane torch heat the whole area and break the bond. JB weld is rated to 500 degrees F and a butane torch can reach 2500 degrees F.

Am I running a risk of permanently attaching these cheap components to my jamis sputnik?
At the moment I can't afford a new bottom bracket and cranks.

Thanks for your help.
The name "JB Weld" is a triumph of marketing. If the product was named accurately "JB Epoxy Glue," I doubt you would even consider it for this application.

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Old 01-22-11, 07:28 AM   #19
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you are better off trying to use a retaining compound on the tapered interface. loctite 609 or similar. these have high compression strength and low shear
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Old 01-22-11, 07:45 AM   #20
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Unfortunately-as all have said- JB weld won't work-despite what JB weld implies"you can machine it etc"- it can't take any shearing force, and it won't really bind strongly to the metal.

Now there might be an adhesive that would fill the spaces and stay in place a bit longer.
Various forms of liquid nails might stay in place a bit longer. Unfortunately, it would take maybe 10 days to fully harden.
Frankly,I doubt it would work-shearing forces are so high( 100 lb ft torque on maybe 1/10" of surface area-actual force maybe 500 lbs actual force and much higher PSI) it would probably break up and crack.

You can buy a used- or even new crankset+BB inexpensively-under $30 maybe.
Luck
Charlie
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Old 01-22-11, 11:45 AM   #21
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If it was mine and i had no money,I would give it a shot.Might work,might not.If it stays tight,great.If it doesn't,so what,your back to square one,big deal.
Just remember, Murphy was an optimist. Patch repairs are to get you back home. Once you're in the warm and dry fix it right so you won't have to do another patch job in the cold rain.
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Old 01-22-11, 11:54 AM   #22
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To hijack the thread just a little bit, epoxy putty adhesives like JB Weld are extremely useful and strong in certain applications. This doesn't happen to be one of them. In fact, I cannot think of any bicycle part that could be repaired with epoxy putty: the stresses are too variable with regard to force and direction. I use the stuff in the auto restoration biz to fix plastic and pot-metal castings, like rear-view mirror housings. It works fine when the stresses are light and in one plane, like a hinge, or where they are constant, such as in trim held on with screws.
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