Threaded crank arm, what's my next move?

sibbouk
sibbouk Posts: 6
edited February 2010 in Workshop
I recently got a MBK rd-100, my first road bike. It has a fsa tempo crankset, however the left crank arm thread has completely gone. I don't seem to be able to find a replacement crank arm for it, and as I am recently new to cycling, I need some help with my next steps I should take. Is it possible to get the crank arm rethreaded? Or do I need a new crankset? I noticed there are varying fitments on the bottom brackets, with mine having the standard square tapered one, but not many cranksets seem to have this fitment. I am also looking to do this for under £50 as am currently at University!

Cheers,
Dan

Comments

  • balthazar
    balthazar Posts: 1,565
    Generic left cranks are about £10: (the link below is ugly- prevents linking to the whole page: click through to page 3)

    http://www.sjscycles.co.uk/category.asp ... =&sfile=1=

    They don't have to match the right hand side, but there may be different offsets, and the square aperture may be aligned differently, so it would be worth checking with the vendor if you go this route.

    EDIT: I think this is the one you want – diamond hole:

    http://www.sjscycles.co.uk/product-Unbr ... -14767.htm

    or black:

    http://www.sjscycles.co.uk/product-Unbr ... -14765.htm
  • Monty Dog
    Monty Dog Posts: 20,614
    Take it to your LBS - they have the know how and the tools to get the old one off and probably have a suitable, low cost replacement.
    Make mine an Italian, with Campagnolo on the side..
  • DaSy
    DaSy Posts: 599
    Maybe I'm just not understanding this, but there is no thread on a left (or right for that matter) square taper crank-arm. The thread is in the end of the bb axle, so I would assume the stripped thread is either the crank bolt or the internal thread in the bb.

    Most likely of those two is the crank bolt as it tends to be softer than the bb axle in my experience.

    Like I said, maybe I'm misunderstanding this entirely...
    Complicating matters since 1965
  • DaSy
    DaSy Posts: 599
    Okay, so I assume this is the part that you thread the crank extractor in then?

    In which case I'd just run it until the BB wears out, and think about an upgrade at that point, as you can just use a standard crank bolt and not worry about the self extracting bit that does nothing in normal use.
    Complicating matters since 1965
  • keef66
    keef66 Posts: 13,123
    I can confirm that. I stripped the threads out of one of the cranks on my son's mtb with the crank extractor. Gave up trying to remove it at that point and just screwed the crank bolt back in. That was 2 years ago, and the BB is still fine.

    (I was trying to get the cranks off cos he'd jammed the chain really hard between the BB and chainring. In the end I belted it out with a cold chisel & lump hammer and replaced the chain)
  • lae
    lae Posts: 555
    A wedge-type balljoint splitter (basically a big two-pronged fork) from Halfords/cheaper motor factor will take off crank arms with stripped threads. Put it between the crank arm and the BB shell and whack with a mallet. They're only a few quid. If you know a friendly car mechanic they might do it for free, it takes about 30 seconds to do.

    If that won't take it off, you can get an angle grinder and cut through the BB spindle, but you'll need to replace the BB obviously!
  • ex-pat scot
    ex-pat scot Posts: 939
    If the crank arm is still attached to the BB spindle, then leave it there.

    if it's off and you want to live with the spider / rings then screw it in and leave alone.

    If it offends you then put a dust cap on to hide the stripped threads.

    It may have been stripped by some monkey trying / failing to remove arm. It happens. Often there's a washer that goes behind the crank bolt but isn't particularly visible. When someone tries to remove crank using a normal thread puller, but forgets to remove the washer, then they bend the washer and strip the threads...
    Commute: Langster -Singlecross - Brompton S2-LX

    Road: 95 Trek 5500 -Look 695 Aerolight eTap - Boardman TTe eTap

    Offroad: Pace RC200 - Dawes Kickback 2 tandem - Tricross - Boardman CXR9.8 - Ridley x-fire
  • dennisn
    dennisn Posts: 10,601
    ex-pat scot wrote:
    It may have been stripped by some monkey trying / failing to remove arm. It happens. Often there's a washer that goes behind the crank bolt but isn't particularly visible. When someone tries to remove crank using a normal thread puller, but forgets to remove the washer, then they bend the washer and strip the threads...

    I take offence at being referred to as a monkey. It wasn't me. I was nowhere near his bike. That was MY bike that I did that to. You're right. It happens. My only consolation after having done it was that "well, there is probably some other idiot out there too". D*mn washers. Man was I p*ssed off when I realized what I'd just done. :oops: :oops:
  • keef66
    keef66 Posts: 13,123
    I'm now wondering if I too am guilty of simian workshop technique. I might have a quick peek tonight to see if there's a mangled washer lurking in there somewhere.

    It would explain my banana habit I suppose

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