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enter image description hereJust bought a Rocky Mountain Dirt Jumper, not sure about the exact model because the previous owner custom-painted it. It is missing a pedal and I noticed that the crankarm wasn't threaded like a normal crankarm, but instead, had some threads that went straight from the top to bottom, and I saw on the other crankarm, there was some sort of adapter that was attached the the crankarm, which also had the pedal attached to it. Wanted to find out what it is and where I can purchase one. enter image description here

Update: Took apart the crankset and I found this, wondering if this works with any types of cranks?enter image description hereenter image description hereenter image description hereenter image description hereenter image description here

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    I’m pretty sure the thread has been stripped (crank is broken), and the adaptor on the other crank is an artifact of a successful repair attempt. Perhaps the previous owner failed the repair on the pedal-less side.
    – MaplePanda
    Mar 15, 2022 at 4:48
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    so i have to buy new cranks/re-tap the cranks?
    – Keyin Liu
    Mar 15, 2022 at 5:01
  • That BB looks like one of the earlier Octalink standards - if you're going to the expense of a new crankset then a new BB might also be in order.
    – Criggie ♦
    Mar 15, 2022 at 20:57
  • i dont have the tools to install an new bb tho, is it possible to just use this bb with a different crankset?
    – Keyin Liu
    Mar 15, 2022 at 21:07

2 Answers 2

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I don't know the model, but the uniformity of the spline pattern in your second picture makes it pretty clear that this is a BMX or other gravity type crank that uses a threaded insert rather than have the threads integral to the crank arm. If so it is proprietary to those cranks. If you can figure out what cranks you have, you might be able to then find a replacement, but it's an uncommon design and looks old so it's also pretty likely you won't be able to come up with much. You're probably looking at needing a new crankset.

The first picture appears to show a 1/2" to 9/16" pedal adapter, hence the oddly long protrusion of the pedal spindle. If that's true, it's a separate piece from the splined insert that these cranks appear to use and doesn't have anything to do with it.

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  • Do I only need to buy a new pair of cranks? Or a new chainring aswell?
    – Keyin Liu
    Mar 15, 2022 at 15:52
  • @KeyinLiu I can't tell what style of chainring is on there. In some cases you could transplant the ring. BMX crank and bottom bracket matching is its own question.
    – Nathan Knutson
    Mar 15, 2022 at 16:44
  • You’re saying there once was a threaded insert that now has fallen out? Interesting. Shimano also uses a steel insert for their gravity cranks, but is it really any more reliable than directly threading the aluminum? 🤔
    – MaplePanda
    Mar 15, 2022 at 18:16
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    @MaplePanda Yeah I would say it's probably much more resistant to issues on average for the application, that application consisting of not always the best mechanical practices at the user level.
    – Nathan Knutson
    Mar 15, 2022 at 18:38
  • Whether it's worth it in terms of the cost or creating a big weird hollow section in the crank is a separate question.
    – Nathan Knutson
    Mar 15, 2022 at 18:43
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Weird - That doesn't look like a helicoil because its not coiled - its straight splines.

I wonder if there's a press-fit bushing that goes inside with an internal 9/16 20TPI thread in left or right handed, and separately the PO has put some pedal axle extenders, also known as Q Factor extenders to push the pedals further outboard.

If you know someone who is a machinist, they might like it as a project. However 20TPI is a unique thread pitch at 9/16" so someone's going to have to buy weird taps, or single-point cut the threads. Possible, but not cheap if you have to pay market rate for bespoke machined parts.

Your other option is a replacement crankset with normal threadded holes for the pedal axles. Used would be fine for this.

Good luck !

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    Yeah, I guess that I'm going to just buy new pairs of cranks and pedals since I don"t think anyone has a tap that can re-thread this crank. Thanks for the help guys
    – Keyin Liu
    Mar 15, 2022 at 15:51
  • so the spindle that I have, the measurement for the part where there are indents are 4.6 inches, so what would I have to do to find a pair of cranks that work with this?
    – Keyin Liu
    Mar 16, 2022 at 4:25
  • Also, would these taps work? walmart.ca/en/ip/…
    – Keyin Liu
    Mar 16, 2022 at 4:26
  • @KeyinLiu Maybe - they are repair taps, intended to chase an existing thread not cutting a new one from fresh metal. With force, lube and luck it should work. According to blacksfasteners.co.nz/technical-downloads/tapping-drill-tables you need a 12mm or 15/32" hole for a 9/16" thread. If your hole is bigger than that, there's not enough metal left to hold the thread.
    – Criggie ♦
    Mar 16, 2022 at 5:08
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    thanks for the info, I think that I'll just buy a new crankset+BB
    – Keyin Liu
    Mar 16, 2022 at 18:33

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