The Golden Wrench

A blog about bicycle repair and maintenance by the mechanics at Freewheel Bike.

Monday, December 13, 2010

Repairing a Stripped Pedal Thread

Occasionally we will have a customer come to us with a right pedal jammed into the left crank arm, or vice versa. In most cases, the pedal has a steel spindle and the crank is aluminum. Aluminum, being much softer than steel will accept the forced thread, but not for long. Once a load is put on the pedal the threads begin to strip out and the crank is toast! Or is it?

Of course, on lower end bikes, a left side crank arm can be purchased for a fairly reasonable price. However, right side cranks or higher end cranks can be much more expensive. In most cases a pedal thread can be repaired.

For science, I went against all of my mechanical training and experience and forced this right (Red for Right) pedal into the left crank arm. It was quite liberating.

Now that our left crank arm is good and wrecked, its time to get fixin'.

Remove the crank bolt.


Extract crank arm.


Up close of the damaged threads. if you look straight down the center, you can see where the newly forced right-hand threads cross the preexisting left-hand threads.


Left-hand in silver, Right-hand in gold. they can also be identified by the direction the threads slope. Left-hand threads slope up to the left and right-hand threads slope up to the right. Same as pedal threads.


Double check the tap to make sure you're using the correct one. The last thing you want to do it tap a right-hand thread in the left crank. This particular thread repair kit uses a special tap with a tapered reamer. It enlarges the hole and then taps the new threads all with one bit. Also, Make sure to use plenty of cutting oil to keep the tap cool and lubricated.


There will be plenty of metal shavings. The tap needs to be sent through all the way.


Pristine new threads!


Grease the pedal threads and install thread insert on pedal spindle.


Apply high strength thread lock compound to the external threads of the sleeve insert. This will keep it from being removed from the crank when the pedal is taken on and off.


Thread insert into the crank. Wipe off excess thread lock compound before threading in all the way.


There will be a bit of overhang on the back side of the crank.


The quickest way to remove this is with a bench grinder.


After the thread lock compound cures overnight, the pedal can be removed and reinstalled as if the new threads were one with the crank.

Time for reassembly.


Grease those bolts! This will allow proper torque to be achieved and protect the threads from corrosion. Park Tool's PolyLube 1000 works great as an assembly grease.


Tighten crank bolt to spec. In this case, Shimano recommends 35 - 50 Nm.


Reinstall pedal.


Here is the finished product. It's hard to tell that there was every anything wrong with this crank in the first place.


Our Shop bike is reassembled and ready for lunch trips to Chipotle once again!

9 comments:

  1. Pete!

    Thanks for this. I actually talked to you on the phone about a week or so ago and performed this (successfully!) on a customer's bike (of coarse, on a dummy crank arm first). Albeit, this post was after the fact, but still very insightful (and reassuring).

    And I'll be sending some post requests your way soon. This is slowly becoming my favorite daily read.

    Thanks for everything and keep up the good work!

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  2. Hey Vincent. Glad that your repair went well. Your call obviously gave us the idea for this post and for taking requests so thank you for the ideas. I'm curious to see how the "request a post" thing goes.

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  3. Thanks for the post.

    It seems though, that there are bits of the process missing. the Gold/Silver inserts appeared as from nowhere and I'd be keen to find out more.

    JB

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  4. This is a great how-to. I'd appreciate knowing where I might find the crank inserts.

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  5. http://www.biketoolsetc.com/index.cgi?id=867204723182&d=single&c=Shop-Aids&sc=Thread-Repair&tc=Complete-Kits&item_id=EL-2299

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  6. thanks a bunch, as a mobile fitness repair tech, this will result in less downtime waiting for replacement parts!

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  7. Hi, this is brilliant and informative. What is the size and thread count of the new bore for the helicoil? The pedal is 9/16 - 20, so I’m guessing the new thread must be something near 5/8 - 24

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  8. I am trying to complete this process on my unicycle. Only thing is that I can't seem to figure out the tap size. It's a left pedal with an original size of 9/16 x 20. Yet the isert seems to have an external size and thread difference. Is it that I need a 5/8 x 24 for the insert?

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  9. Regarding the tap size questions:

    The internal threading on the inserts is standard pedal threading of 9/16"-20. The external threading is necessarily a larger diameter, but it is also a different thread pitch at 24tpi.

    I took some careful measurements of the threaded inserts and also the special Unior piloted tap/ream. The only way to properly measure and compare threaded parts is by their pitch diameter, which I measured using Pee Dee Thread Measuring Wires and a micrometer. Here's what I came up with:

    MAJOR DIAMETER:
    tap = .630"
    insert = .629"

    PITCH DIAMETER:
    tap = .607"
    insert = .601"

    This is absolutely not a 5/8" thread size, since according to Machinery's Handbook, 5/8"-24 must have a pitch diameter between .5927" and .5967".

    If there were such a thing as M16-24tpi metric thread with an inch pitch, this might be close. I didn't feel like spending 20 minutes calculating the theoretical pitch diameter limits from formulas and tables since you're not going to find such a tap anyways, and especially not a left-hand tap.

    The moral of the story is that the only way to properly install these inserts is to use the official tap from Unior, since the thread size is proprietary. This also gives you the benefit of getting the tapered reamer and pilot to enlarge the hole to the correct size and align/guide the tap as it starts. For those who want to do it the hard way, you'll need a 24tpi tap with a suitable pitch diameter, and you'll need a way to enlarge the damaged hole to .568" (the diameter of the parallel section of the ream/guide on the Unior tap) while maintaining the hole's alignment relative to the bottom bracket's axis of rotation.

    We have the proper taps and can perform the entire repair for $21 labor per crank arm, plus the cost of the threaded insert. You'd just need to get the crank arm to us either in person or by mail.

    Karl Stoerzinger
    Freewheel Midtown Bike Center

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