A Jig For Shortening Cranks- Mtbr.com
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  1. #1
    mtbr member
    Join Date
    Dec 2007

    A Jig For Shortening Cranks

    I've dug up all the threads I could find about shortening crank arms and the one thing I can't seem to find is a photo or good description of a jig to make sure that the new hole is drilled straight. If there is a thread with this info...I totally missed it.

    I thought about it for a bit and came up with this. I brought two pedal spindles to my local welder and asked them to weld them to a rectangular tube. They drilled a hole in the top and bottom so the spindles were perfectly straight, then tacked them in. Now I can put the crank on the jig, clamp the jig to the drill press, and go for it.

    I'm waiting for the thread taps to come in the mail before I give it a try. I'm gonna hit the dumpster at the community bike shop for some practice cranks as well.

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    A Jig For Shortening Cranks-img_8224.jpg

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    A Jig For Shortening Cranks-img_8226.jpg

  2. #2
    mtbr member
    Join Date
    Feb 2016
    First set I did a made a jig... the last 4-5 I didn't bother.
    Once you have the pilot it is easy within detectable tolerances on the pedal is in.
    I found controlling drill speed is a bigger issue in soft alloy and HSS bits....especially as the size gets bigger and you generate enough heat to start damaging the annealing process.

    Tapping is even easier... I used 3 forwards, 2 whole back and some thick grease to clear swarf ...

  3. #3
    mtbr member
    Join Date
    Dec 2007
    The taps came in the mail. The jig worked like a charm! I had to put a 2x4 scrap and a shim or two under the crank arm while drilling it so it wouldn't flex from the force of the drill press but it worked out really well.

    As for tapping, I didn't know the best way to make sure the tap was straight. I did the first arm with a speed square next to the tap and I checked it like 1000 times while tapping. This was not that easy. The second one, I clamped the tap in the vice upside down and I threaded an unaltered crank arm on the tap a few turns. Then I stacked the newly drilled crank on top of that and I turned them at the same time. This worked really well, and it was WAY easier then using a square.

    I used the parts of the cranks that I cut off with a hacksaw and traced the radius over the new holes, then took it to the bench grinder. All worked out really well!

    A Jig For Shortening Cranks-e5e366f4-a8cf-46c2-9f74-2a3603bee506.jpg

    A Jig For Shortening Cranks-img_8261-1.jpg

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