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Cotter Pins
Dear Jonny,
You've pre-empted my post!
It's my own fault for being slow, but the fact is I've found it more difficult than I expected to put the process into words - it's the sort of thing that's best learnt from a demonstration in the flesh. However now that Jonny has broken the back of the problem I'll try to add a few helpful hints.
Filing
I guess this is the tricky bit - it's a low tech skill and something we're not very used to doing nowadays.
You will need a good file (sharp, but not too coarse) and a vice, preferably firmly bolted onto a bench. In an emergency you might hold the cotter in a Mole wrench, but this is not recommended.
It's possible to be lucky and have cotters that just go straight in without filing, but this is unusual and normally you will find the cotters need to have some material removed so that they go far enough into the crank to allow sufficient thread projecting to take the securing nut. Unless you're careful you will alter the slope of the pin's flat (or you may well have started with a pair of cotters that weren't the same to start with) and this will cause the cranks to go out of line. Now it will be necessary to adjust the angle of at least one of your cotters, and this is not so easy to describe - I suggest you look down on the non straight cranks and picture in your mind's eye whether the slope on the offending pin is too shallow or too steep, then put that cotter back in the vice and file it to achieve the desired alteration in the slope of the flat.
The 'offending' cotter is the one with most 'meat' on it - the thicker one, because it's possible to file away so much metal that there's no thread left projecting beyond the crank.An elementary mistake is to put both cotters in the same way round, which will certainly put the cranks out of line. I'm sure no one on this forum would do this, but I have seen bikes being ridden with this fault.
As Jonny says, the continental practice is the opposite from the British method, which is: crank forward, nut upwards. I've seen a lot of discussion about why one way is better than the other, but I note there a lot of very well used bikes around which have both methods, and I conclude that it really doesn't matter.
When reusing old cotters, there is often a burr (or witness mark) where the pin has been in contact with the spindle - remove this with your file so that the flat is flat, but don't take off more material than is necessary.
When fitting the filed cotter pin, again as Jonny says, supporting the crank is very important. I like to put some thin oil on the mating surfaces because I believe this helps the pin to 'settle'. There does usually seem to be some settling in, and that's why it is essential to check for tightness after a couple of miles and then again after a longer ride. Once it's firmly settled your cotter pin should stay in place for decades of use, and still be removable and reusable by future generations of old bike users.
I had hoped to pass my knowledge on to a future generation, but out of three children and the first two grandchildren I have been quite unable to stir up any interest in the subject. However the third grandchild is at least beginning to show some promise. I have a photo of her aged about two apparently about to attack a very small child's trike with quite a large spanner - I would have posted it here, but pictures of small children have a bad reputation on the internet. I will post a tamer pic in a few minutes when I've found it.
clubman-
I didn’t mention filing. I think I’ve been lucky the last couple of times that I haven’t had to do it. Like you say, it’s a very easy thing to get wrong. Another thing that came to mind when I was reading your post is before filing the flats it’s worth checking where the cotter is actually catching. Sometimes you just need to file the edges of the flats so it clears the slot in the axle and then it drops correctly into place. Again, you want to take the absolute minimum of material off so that you don’t affect the crank location.
Jonny69
Installing cotters:
Doesn’t matter which way the cotter goes in. Crank forwards / cotter down or crank backwards / cotter down. One is the English way, the other is the Continental way. One way your flares will get tangled up on the nut, the other way they won’t. I can’t remember which is which.
Support the underside of the crank as Clubman did or with a stout piece of pipe between the crank and ground. Cotter goes in with the flat against the axle flat. Wriggle it down until it seats. Give it a couple of sharp blows with a hammer and tighten the nut. DO NOT use the nut to tighten the cotter in because it will shear. Do a short test ride and check it, repeat the above if necessary.
New cotters aren’t as good as old ones. If you get your old ones out in good condition I recommend re-using them. Next best is NOS old ones. For new ones, you’ll see that some of them look machined and some of them have a smooth rolled finish. Choose the latter, as the former are just machined low grade steel and are too soft and weak. Definitely get 3/8” cotters if you can, if they’re Imperial, for the reasons Clubman made.