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  1. #1
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    Trek 3700 crank arm stripped

    So I ride a 2006 Trek 3700, which has been awesome for me until today. I rode a bit yesterday and the crank arm on the sprocket side belt like it might have a bend in it. Turns out it wasn't a bend in the crank, but my pedal had stripped out all the threads. It was an interesting ride home to say the least.

    Anyway, my question is what I would need to upgrade to higher end crank arms. Would I need to get the whole assembly or could I just get the arms? The crankset it's using is an SR Suntour SR XCC-T208 48/38/28. I'd like to upgrade to an LX set if possible, and if not, I'd like to at least replace it with the SR Suntour junk again, although I'm having a hard time finding any online.

    Thanks in advance for any advice, and my apologies for any MTB terms I've butchered.

    -Pete

  2. #2
    Lionel Hutz, Esq.
    Reputation: Thirdrawn's Avatar
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    The short answer: Yes, you can just replace the crank arms. Unfortunately, your upgrade options are fairly limited at that point.

    The cranks on a 3700 are a square-tapered design. I call Suntour's design a "diamond" because the square hole is rotated 45deg from the Shimano design. While the difference is purely asthetic, it means that the two systems don't work together.

    2007's LX cranks are a fancy two-piece design. The older LX cranks use an "Octalink" system. Again, this system is not compatible with what have on your bike.

    Basically, in order to get much of an upgrade, you'll need to get new crank arms and a new bottom bracket (the part that holds the crank arms together).

    If it were me, I would get a replacement part on par with what you had and save the money you would spend to upgrade and use that to get a new bike. Even if you get basic entry-level stuff, you'll probably spend around $100 and that's roughly a third of the cost of your 3700.

    In any event, your local bike shop can give you lots of options. Sorry if this is discouraging.
    2007 Trek Fuel EX 8
    1999 Trek 7000 --- Dragonfly Green = Sexy
    2002 Trek 2100
    2008 Felt New Belgium Cruiser

  3. #3
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    Not discouraging at all, thanks for the reply. Just wanted to know what I was getting myself into. I definitely think I'll just replace with the SR Suntour stuff again, and just start stashing away money for a $600-700 range bike.

    Thanks again

  4. #4
    mtbr member
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    This is a situation similar to what I'm looking at right now. I've got a 2006 6500 hardtail and I just bought a 2008 EX8. To keep it simple, my cranks on my 6500 have got some problems so I was thinking of moving the LX cranks from my EX8 to the hardtail and then getting a new set of XT cranks for the EX8. I realize I have to change the bottom bracket as well.

    Questions...
    Would the external bearing bottom bracket from the EX8 fit on the 6500? Are there any compatability issues?

    OR... Should I leave the bottom bracket in the EX8 and just get a different bottom bracket when I get the XT cranks? I could then put this new BB in the 6500?

    Do cranks automatically come with a bottom bracket or do you have to buy it separately. IF they come with, is the bottom bracket that comes with the XT cranks better than what would have come on the EX8 with the LX cranks or are they the same?

    Hope that isn't too confusing? Thanks for any help.

  5. #5
    Lionel Hutz, Esq.
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    Unless you're buying a two-piece system (integrated BB design), the cranks will be a separate purchase from the BB.

    Generally, BB's are pretty compatible. You shouldn't have a problem with the EX8 or the 6500. The only issue will be whether you have a 68mm or a 73mm BB shell. Most BB or two-piece cranks will come with two 2.5mm spacers so you can run your new BB on either shell width. I feel it's prudent at this point to mention that some cranksets will require a certain type of BB (square-taper, ISIS, Octalink, etc)

    Answers...
    The external bearing bottom bracket from the EX8 will fit on the 6500, but you might require a spacer which you can get at your LBS.

    You can buy some new cranks if you want, but you don't need to.

    Just before I got my EX8, I got a new two-piece crank for my 7000. When I got the EX8, I took the stock cranks off and switched it with my 7000's new cranks. Basically, you can swap stuff out to your heart's desire.

    If I were you, I'd look at what kind of upgrade I could get on a new crank. If it was a good upgrade, I'd put the LX cranks on the 6500 and put the new, upgraded cranks on the EX8.
    Last edited by Thirdrawn; 10-29-2007 at 02:24 PM.
    2007 Trek Fuel EX 8
    1999 Trek 7000 --- Dragonfly Green = Sexy
    2002 Trek 2100
    2008 Felt New Belgium Cruiser

  6. #6
    mtbr member
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    Thanks Thirdrawn.

    That's what I was hoping to do when I mentioned buying new XTs. The existing LX cranks from the EX8 would go to the 6500 and I'd put new XTs on the EX8.

    I just wasn't sure about different widths on the cranks and how it might affect chain alignment. I noticed looking around that there are different cranks available but now I realize how dumb that was... those are the crank arm lengths 165, 170, 175, 180.

    Anyway, I guess what I'd have to look at doing is getting a BB that will fit the 6500 and still be compatible with the LX cranks and just get the XT cranks and keep the existing BB on the EX8.

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