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.
“As a bonus, pedal spindles are usually hardened, so they make perfect thread chasing tools, even in steel crank arms.”
But Harv, if you remove the temper in order to file the requisite notches in the spindle threads, doesn’t that negate the above statement?
In any case, your idea is a case of sheer and unadulterated brilliance!!! I absolutely love the idea, and am stripping old pedals as we speak so I can make my own tools!
Ah, yes, you are correct, an apparent contradiction. However, I will now try to weasel out it!
Hardness of steel is not an all or nothing proposition. There are varying degrees of hardness, as Mr. Brinell tells us with his scale of hardness. As I said above, different spindles have different degrees of hardness. I have found that the hardness varies along the length of the spindles. Probably because the manufacturing process heats the bearing area locally (I would guess by induction) and then quenches the whole spindle, imparting less hardness at points remote from the bearing area.
I had no trouble filing flutes in the right-hand spindle without removing any tempering. However, the left hand one (from a different pedal set) would resist all my attempts at filing. In such a case, just enough hardening must be removed to allow the file (which is also hardened) to cut the flutes. My method of annealing is inexact, you takes yer chances. Hopefully, enough hardness remains to strengthen the cutting edges of the flutes. For a thread repair in an aluminum crank arm, this point is moot. For a steel crank arm, you hope for the best.
Harv, I think you have successfully weaseled out…a rational argument for sure!
Since most of the crankarm threads I would be chasing are aluminum, I guess I don’t need to worry about the hardness (or lack thereof) of the spindle threads.
Is there any “home” way of returning the hardness to the spindle? Quenching in oil, or air-hardening, or something? I suppose it doesn’t really matter, but if you have a technique for re-hardening, I am all ears!!!
Wow! We ARE getting into the heavy technical stuff, aren’t we. I have actually thought about this before. As you said, re-hardening is a matter of heating up the metal to somewhere around 1000 degrees F (this is a dull red color for steel) and suddenly reducing that temperature by quenching in a cool liquid. As I remember it, quenching in oil is done to get some carbon into the hot steel, which will alloy it into a harder and stronger metal. I doubt you can do this with a propane torch or a stove-top. Probably at minimum, a ‘blow-torch’ is required to get the metal up to that temperature. A hobbiest ceramic oven comes to mind as a good way to do the job.
But you can’t go wrong getting it as hot as you can with whatever you can and quenching in either oil or water. This might not be ideal, but should result in a harder material than resulted from the annealing process.
While heating the spindle with a torch, it may be good practice to use some sort of insulator between the spindle and the vise (or pliers, or vise-grips) that you’re using to steady the piece. I’m thinking maybe pieces of silicone rubber?
If you hold it directly with metal jaws, the vise or pliers will act as a heat-sink, pulling heat away from the steel spindle you’re trying to get up to temperature.
Just my .02
Valid point. To minimize the heat sink effect, I have relied upon clamping the work as far as possible from the area to be heated. Also, the small contact area of the clamping device helps. This is how I handle soldering and brazing as well. Ideally, of course, insulating the work from the heat-sinking effect of the clamping device is called for. Unfortunately, heat insulators are often combustible (wood, cloth, plastic, etc). Silicone rubber sounds like a good way to go.
Wow! Great story, Tes. Your method was very resourceful, you didn’t have a propane torch, a workbench, or a vise, but you made do with what you did have, and affected a repair that your LBS could not handle. Quite an accomplishment!