Make Your Own Pedal Thread Chasers

Author: Harv

As a followup to this month’s Nuts ‘n’ Bolts on French thread bike pedals, here is another pedal/crank problem solving issue. If you are not careful threading pedals into their respective crank arms, damage to the internal threads may result. If the aluminum threads are not actually stripped, they can be repaired using a ‘thread chaser’. As far as I know, no tool company makes a 9/16 x 20 right hand or left hand thread chaser. There are two British thread forms that are now standard for bicycle production worldwide. These are the 20 and 26 TPI (threads per inch) CEI (Cycle Engineers Institute) thread forms. There are 20 TPI taps made for pedal threads, at a cost of about $35 a pair. For the home-mechanic, this is a bit pricey for just occasional use. Here is what I have come up with, at a cost of zero.

Almost everyone has a pair of unused pedals laying around. New bikes come with those cheap plastic toy pedals that just serve to fill the holes in the crank arm until you can install those spiffy clipless jobs. Used bikes hardly ever come with the pedal that you want. So I hope you have not tossed out those junk pedals, we are going to need them for this project.

First, strip the pedals down to the spindles. (“Strip” is wrench-talk for disassemby). There must be good threads on the spindles. Clamp each spindle in the vise, grab a three-cornered file and make four tapered notches, 90 degrees apart, as shown in the photo. Each notch serves the same purpose as a flute on a tap. That is, the notch, or flute, will form cutting edges for scraping out the thread damage and reforming the thread in the crank arm, and will also accumulate the resultant metal chips. As a bonus, pedal spindles are usually hardened, so they make perfect thread chasing tools, even in steel crank arms.

Right-hand Thread Chasing Tool Made from Pedal Spindle. Shown Atop Now Famous Bike Oven Aluminum-Jawed Vise
Photo by Josef

Another View Showing All Four Flutes
Photo by Josef

A slight diversion into metallurgy is called for at this point. Since pedal spindles contain bearing surfaces, they should have been hardened to some degree to resist wear. Sometimes only portions of the spindle are hardened, sometimes all of it, sometimes none of it (cheap pedals). If the spindle you are working with seems to be unfazed by your file, the hardening will have to be removed to proceed. To do this, clamp the spindle at the end opposite the threads. Heat up the threaded area with a propane torch (or stove). Just apply the hot part of the flame (the bright blue cone near the tip of the torch) for a couple of minutes. If you see a thread turn dull red, you have heated it enough. Let it cool to ambient temperature slowly. Do not blow on it, put it in water, or anything else to make it cool fast. Slow cooling anneals it and removes the temper, or hardness. Then file in the flutes as indicated above.

Use this home-made tool as you would a proper thread chaser or tap. Use cutting oil (or machine oil) generously on the tool and the threads to be repaired. Carefully insert it to get it started straight, turn clockwise (for the right hand crank arm, counter clockwise for the left one) about a half or one third of a turn, then reverse direction until the tool loosens up. This is to allow any chips to accumulate in the flute as described above. Use a 15mm wrench on the flats of the spindle to gently turn the tool. Don’t force it, you can do more damage if you use more torque than is necessary to just chase the damaged threads. Proceed like this all the way through the threaded hole. It may be better to start on the other side, the inside of the crank arm. Each job must be evaluated as to where the damage needs to be repaired.

Be sure to clean all metal swarf off the tool and repaired threads when complete. Use silicon grease on the pedal threads and reassemble them into the crank arms. You just saved yourself a hunk of cash, lots of time, and learned a bit about metallurgy and working with threading tools.

For external threads, like those on the pedal, I recommend a thread file. These are sold at tool specialty stores and are much more practical than finding all the correct dies or chasers for odd threads such as pedal, fork, hub, and bottom bracket parts. Common small size thread chasers such as for axles, binder bolts, and miscellaneous small fasteners on the bike are cheap and readily available. You could also make thread-chasers by notching nuts internally with the same three-cornered file as we used on the pedal spindles. Photo below shows files for both metric and US threads.

Thread Files from Snap-On, Metric and U.S. Sizes Shown

So, here are the world’s first bicycle crank arm thread chasing tools. Custom machined and available at the Bike Oven.

Make Your Own Pedal Thread Chasers