Like many parents on this forum, I'm a fairly avid cyclist and have accrued parts over time that are no longer in use. I've also accrued children over time who could maybe put said parts to use. Problem is grown-ups = big, and kids = small.
So we've been steadily making little tweaks and changes to my son's specialized hotrock, and I wanted to get him a better crank setup. I had an old Cannondale Coda crank i got off e-bay for like $30, complete with chainrings and BB that had been taken off another project and I just didn't see them being used, so I decided to try and adapt them for my son. Being a fan of accruing tools, i found a reasonable set of pedal taps off ebay for like $35, and purchased a 9/16 drill bit (although a more common 1/2" will also work, just be harder to tap).
All i used was a cheap power drill, hacksaw, dremel, file, a wood block, and my son helping hold things steady.
Step 1) Eyeball desired crank length (i believe we went with a 135? About 10mm longer than what he had). Shown here is the original 175 vs. the new holes. The wood block was used to back up the crank as i drilled into it. Get as plumb to the crank as you can :-).
Step 2) Trim off the extra arm. I think I actually used my chopsaw here, but a hacksaw would have been just as good.
Step 3) shape to clean it up. I cut the square corners off with a combination of the hacksaw, dremel, and a file. The dremel didn't actually help too much. Just gave the sawing muscles a bit of a break.
Step 4) Install and enjoy. They are reasonably light, plenty strong, roll on a decent BB, and look pretty cool (although he wants to paint them electric blue).
The keen eye will notice that this particular crank is thinned out on the backside, but there was still enough meat for 3 or 4 threads, which is enough as long as you don't super crank down on the pedals tightening them.
His next set may be an old pair of race face cranks that have been sitting in the parts bin forever...
This was surprisingly easy to do, and any "not straightness" in the drilling hasn't been an issue.
Just thought I would throw this out there and share with the universe. I now have no concerns about making more "custom" cranks as he (or his sister) gets older, and now I have the tools!
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Thread: DIY crank shortening