Wondering what type of pedal/adapter I need for these crankarms

Question

Just 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.

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

Answer 1

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.

Answer 2

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 !