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Old 10-04-09, 03:32 PM   #1
nellyeff
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Bottom bracket or Chainset?

Hi guys, and girls of course....

I have a problem with my Giant, the left side crank arm keeps coming loose. Its a tapered square drive axle BB, and a Truvativ Isoflow chainset.
I've torqued the bolt up and used threadlock, but it still came loose.
I tried changing the stainless steel 8mm hex key bolt for a 14mm mild stell one to see if it had any more bite but no joy. I've also tried tightening the bolt way beyond its recommended torque, but it still comes loose.
The threads on the bolt seem fine, and I have no problem with the fixing slipping or crossthreading when I'm tightening it up.
I obviously need either a bolt or a BB but which is it????

Thanks in advance.
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Old 10-04-09, 03:43 PM   #2
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You need a new left crankarm.
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Old 10-04-09, 03:59 PM   #3
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A new arm?
Do these wear? I noticed its alloy and my axle is stainless, I take it the alloy softens up causing it to "wiggle" loose?
Sorry if I'm sounding contradictory, its not intended that way. It never even crossed my tiny mind that it could be the arm itself, but it certainly makes sense the more I think about it.

Thanks for your speedy reply.
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Old 10-04-09, 04:06 PM   #4
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Correct. If it's been repeatedly coming loose with the proper torque and you've been riding it loose, the square hole has started become rounded. It will no longer ever stay tight on the axle. The arm is toast. The good news is that replacing the left arm is significantly cheaper than the right.
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Old 10-04-09, 04:08 PM   #5
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Indeed it is, thanks plenty for your help mate.

*Heads for Ebay....*
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Old 10-04-09, 04:16 PM   #6
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Indeed it is, thanks plenty for your help mate.

*Heads for Ebay....*
Make sure you get the same arm, they take a proprietary system called power spline for the interface.
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Old 10-04-09, 04:19 PM   #7
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Correct. If it's been repeatedly coming loose with the proper torque and you've been riding it loose, the square hole has started become rounded. It will no longer ever stay tight on the axle. The arm is toast. The good news is that replacing the left arm is significantly cheaper than the right.
Yep. Generic replacement left crankarms are usually less than $15 at most bike shops. The trick part is getting enough torque on the bolt the first time to keep it from coming loose and wallowing out again. 25 to 35 foot-pounds of torque is needed, and that's tough to do without a dedicated wrench. I don't like the Allen-key crank bolts since it's darn near impossible to get enough torque on them. Nelly had a good idea with replacing the Allen-key bolt with a regular bolt, but it was probably already too late.

Here's the Park page on square-taper cranks: http://parktool.com/repair/readhowto.asp?id=88
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Old 10-04-09, 04:22 PM   #8
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Make sure you get the same arm, they take a proprietary system called power spline for the interface.
Maybe, maybe not. From what I can see, the Truvativ Isoflow stuff comes in both square-taper and Power Spline. Since Nelly said it's square-taper, I'd assume that a generic replacement would work.
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Old 10-04-09, 04:37 PM   #9
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Yep. Generic replacement left crankarms are usually less than $15 at most bike shops. The trick part is getting enough torque on the bolt the first time to keep it from coming loose and wallowing out again. 25 to 35 foot-pounds of torque is needed, and that's tough to do without a dedicated wrench. I don't like the Allen-key crank bolts since it's darn near impossible to get enough torque on them. Nelly had a good idea with replacing the Allen-key bolt with a regular bolt, but it was probably already too late.

Here's the Park page on square-taper cranks: http://parktool.com/repair/readhowto.asp?id=88
Not sure where this rant is coming from. 8mm hex bolts are fine.
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Old 10-04-09, 04:41 PM   #10
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Maybe, maybe not. From what I can see, the Truvativ Isoflow stuff comes in both square-taper and Power Spline. Since Nelly said it's square-taper, I'd assume that a generic replacement would work.
Yeah I think i'm losing it. You're right.
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Old 10-05-09, 12:15 PM   #11
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I use a 3/8" torque wrench for the bike, I'm a bit anal with things like torque settings which I always think are there for a reason. Comes from restoring/modifying cars most of my adult life I think!
I changed to a bolt because I felt a bit more confident giving it a bit of "weight" in an attempt to tighten it up more.

As it turns out I'm covered by the bike warranty, I phoned my bike supplier today and he agreed it was the arm. Send it back with the receipt and he'll send me a new one.
Thanks again.
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Old 10-05-09, 05:48 PM   #12
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Yeah I think i'm losing it. You're right.
If you ever find it, let me know. I think mine's in the same place.
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