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Bent Crank arm - drive side.

bsb1971bsb1971 Posts: 19
edited February 2015 in Workshop
Bought a 2nd hand set of Dura Ace 7800 cranks on Ebay awhile back, only to discover today that the crank arm on the drive side is slightly bent. When fitted and switch to big ring, the crank arm is actually under the chain. We used a couple of spacers to move the shaft out slightly, it is okay until you move the chain onto the bottom ring with the chain on the big ring, the arm slighly rubs off the chain. Is is possible to get the crank arm straightened? Or have I just learned an expensive mistake?

Posts

  • Firstly, I'd check whether its the chainring or some other chainline issue rather than the crank which is bent.

    If it is the crank, I'm going to vote "its a gonner".

    You might get away with bending it back, but Al isn't terribly fatigue resistant so it might crack. I don't rate your chances of bending it back so that the padal spindle is straight, either (which can't be the case now, btw). Also hard to imagine how you are going to get enough leverage on it to bend it back without damaging something else.
  • Thanks for the response. Its definetly the crank arm, just took them off and put on my old ultegra crankset, works perfectly. Compared the drive side crank on the dura ace to the ultegra, can definetly see that the dura ace is definetly slightly bent. I'm definetly going to try straightening it, more out of stubbornness than anything else - p**sed off that someone would sell it as working ok when it isn't though.
  • DKayDKay Posts: 1,652
    I wouldn't bother. Just contact the seller or open up a case via ebay/paypal. Trying to bend it back is a bad idea because:

    If you make it worse, then you haven't got a leg to stand on in terms of getting your money back.

    As already said, aluminium doesn't like being bent and you'll fatigue the crank. You could heat-treat it afterwards, but do you have the facility to do that?
  • keef66keef66 Posts: 13,123
    Ain't these things meant to be hollow? I think trying to bend it back will only end in tears. If you think it's too late to raise an Ebay dispute...

    viewtopic.php?f=40004&t=13018110

    Why not get together with this chap who it seems suddenly needs a NDS crank arm!
  • Thanks for the replies, on the ebay open a dispute, its too long since I bought it (bought it last year, I'd been carefully putting together a 7800 groupset), I do intend digging out the guys contact details on ebay to give him a piece of my mind though, won't do much, but at least i will get to vent.
  • imposter2.0imposter2.0 Posts: 11,025
    Are you sure it's bent? They're not the sorts of things that usually get bent - except in a crash maybe. And if it was in a crash, you would probably see some evidence of damage.
  • deswellerdesweller Posts: 5,271
    bsb1971 wrote:
    Thanks for the response. Its definetly the crank arm, just took them off and put on my old ultegra crankset, works perfectly. Compared the drive side crank on the dura ace to the ultegra, can definetly see that the dura ace is definetly slightly bent. I'm definetly going to try straightening it, more out of stubbornness than anything else - p**sed off that someone would sell it as working ok when it isn't though.

    Are the Shimano chainsets multi-piece like Rotor chainsets then? I thought the crankshafts were an interference fit in the spider on Shimano.
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  • mamba80mamba80 Posts: 5,032
    Imposter wrote:
    Are you sure it's bent? They're not the sorts of things that usually get bent - except in a crash maybe. And if it was in a crash, you would probably see some evidence of damage.

    Pedal takes the damage and presto bent crank arm with no sign of damage, got a 5700 with very similar - just be double sure its not one of the spider arms thats bent, we ve straigtened out one of those before.

    the Da 7800 is one peice hollow crank arm and spider, separate chain rings.
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