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I picked up a nice vintage bike, the kind that still uses cottered cranks. The left crank was loose when I got it, and the pin is completely toast. Replacements are available and very inexpensive, and I'll run out and get a replacement tomorrow morning. But, as Sheldon Brown noted:

If the bicycle is ridden with loose cotters, the motion of the crank will cause the axle to cut notches into the cotters, and it will become impossible to tighten them.

Indeed, the pin was loose. I could hammer it in and it would remove play from the crank arm. I punched the pin out, cleaned and re-greased the BB, and noticed the axle is indeed damaged:

enter image description here enter image description here enter image description here

After re-assembling the BB, I can't seem to use the same pin to tighten the crank onto the axle. Maybe I'm trying from the wrong side, seeing as one end of the axle (second picture) is worse off than the other. It's likely that the pin is too damaged to be of any real use.

What worries me is the damage done to the axle, which, together with the drive side crank and chain ring, forms a single piece: The axle cannot be replaced without also replacing the right crank and chainring, which is expensive (at least in terms of the purchase price of the bicycle). The question, then: Is this cottered bottom bracket axle still of use?

I suppose I'll find out tomorrow when I'll have gotten the replacement pin, but the curiosity and anticipation are killing me. Plus, I can't seem to find any similar discussion online.

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    Lots of useful information here - sheldonbrown.com/cotters.html
    – mattnz
    Aug 16, 2022 at 0:39
  • 1
    Having quoted that site, you'll find I've already visited. But thank you for providing a source :)
    – jayded-bee
    Aug 16, 2022 at 10:16

2 Answers 2

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I would say "it is what it is" and you can't do anything to improve the axle now.

Try the new cotter pins when they arrive, and if it doesn't work, then you can investigate deeper and have more info.

I'd recommend doing nothing at this point. Specifically, do not file anything. See how the pins fit and then make some decisions.

The only thing you can do is clean it - working on a clean bike is much nicer than working on a dirty bike. So wash it or at least wipe down the tyres/rims/frame, and remove the chain for a good multi-day soak in degreaser if you plan on reusing the chain.

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    Pictured is the bottom bracket area, cleaned as well as I can be bothered to do with my dirty rag. Wiped only because I decided to do work around it. If it was mine, I'd disassemble it to bits, clean it until it shines, then reassemble while replacing the standard wear-and-tear items. This one isn't mine, and I wasn't asked to do any overhauling, so I'm limiting myself to doing only whatever is necessary to make it roadworthy, thus the whole bother with the floppy crank.
    – jayded-bee
    Aug 16, 2022 at 10:18
  • Wrt filing the axle, you're invited to take a look at the answer I've cobbled together. The first image shows the pin is pushed pretty deep into the hole in the crank. I think filing the axle flat down to correct its angle might allow the pin to be pushed too far in, rendering the axle completely. The solution isn't perfect, but it is "good enough" for the next couple of months while the owners decides what to do next. Maybe a solution could be to weld a bit of material onto the axle flat?
    – jayded-bee
    Aug 16, 2022 at 15:20
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The new pin is installed, here is the final product:

enter image description here

The axle is damaged to the point that the pin has to be driven much too far in, and the retaining bolt is useless because it can't touch the crank. (This is not visible on the above image.)

I'm sorry for the blurry picture, but I want to illustrate a point beyond the fact that the arm is installed: Notice the angle between the drive side crank and the NDS crank is not 180°. I assume this is the result of the damage done to the axle. Note, this is my second time working with cottered cranks; Outlined ahead are my assumptions that may or may not be correct.

The damage done to the axle, as seen on the images in the question, have changed where the "flat" part of the notch lies. The forward edge of the axle notch has been worn away, due to being pressed on by the loose pin during pedaling. This has changed the tangent of the flat surface on which the pin's flat surface (the wedge) rests. Following is an exaggerated illustration of what I believe to be the case, looking at the axle from the NDS:

enter image description here

The left part shows the axle flat as it should be when new -- It is at a 90° angle from the DS crank. The force of pedaling in this image is counter-clockwise, so the flat is worn away on the left edge (or, if sitting on the bicycle, on the edge closer to the front wheel -- the front or forward edge). The result is seen on the right part of the illustration: The angle of the flat (relative to the DS crank) has been changed, and the tangent has been "rotated" by a few degrees CCW. Thus, the angle of the NDS crank has also been shifted a few degrees CCW.

Trying to install the pin from both sides of the NDS crank, I have noticed that the offset angle (between the DS and NDS cranks) is greater if installing the pin from the rear than if installing from the front ("from the front" = positioned as pictured in the first image in this answer). This makes sense, as the non-flat part of the pin must be perpendicular to the NDS crank, and the direction of the slant on the pin (of the wedge) makes a difference in the angle of the NDS crank relative to the DS crank, as illustrated below:

enter image description here

This also begs the question of whether a brand new axle flat is indeed completely perpendicular to the DS crank, but as the only cottered BB axle available to me is damaged, I cannot answer that question. (Also, this two-piece arrangement of (1) cottered DS crank+axle and (2) separate NDS crank seems to be a bit of a unique thing on these bikes, as common as they may be where I live -- Most searches for "cottered BB" online return results of a three-piece system, where both cranks are attached with pins, and the axle is easily replaceable.)

This answer is long enough and could probably be worded and illustrated better (dear reader: you're welcome to try), but the bottom line is that the axle pictured in the question is good enough to ride in the city, provided the rider isn't bothered by the slight difference in crank arm angles. The rider will also have to take great caution not to exert too much force on the pin and keep an eye on whether it goes loose, as the retaining bolt now serves no purpose. The pin is only held in by friction/stiction. As illustrated by the opening quote by Sheldon Brown in the question, this is what happens to the axle when you ride with a loose pin. If the axle was damaged any further, the pin would simply slide through the hole in the crank: This axle should be replaced in the near future.

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