B***** cotter pins!

[Uncle Monty]

And breathe......

Right. Thats the first one destroyed then. Duprat cranks, so I am assuming 9mm rather than 3/8 pins. The holes in the arms measure 9.2. So I search around and order some 9mm pins. They turn up and I measure them. 9.4 mm. On the phone - no, I am assured by the guy they are 9mm and he ordered them in specially. 3/8 is about 9.5 so hey- maybe he is right and they are a bit oversize. Now I have a fairly good idea on fits, and I am pretty sure that a soft pin is never going to stand a chance of being driven into a hole with around 8 thou interference. Not and end up looking sensible at the end of it! So I took a couple of my new pins and ground them down accurately to 9.25 and gave it a go. Not a chance - the damn thing started to twist as I drove it in and the thread barely emerged from the other side before the end turned into a mushroom. Crank was supported on my anvil - and I was using the Bad Mutha - a 24oz ball pein hammer with a 16" handle. It always wins.
So my question - what sort of a fit is a new cotter pin in the crank arm hole? Should they be a snug clearance fit in the arm before assembly?Without a pin, the cranks are a fairly loose fit on the axle which is new. I knew I should have bought that Mercian instead....

 

[Robbied196]

I feel your pain :roll:

A photo would be good but it sounds like you have been given the wrong size. They should push fit by hand enough for the thread to poke through by a couple of mm. Sometimes you need to make sure the flat of the spindle is rotated slightly, so that the flat edge of the pin can slide in. Once its through tap in with a hammer, tighten the nut, ride a few miles and tighten again. You also need to make sure if the nut is at the bottom on one side, it is at the top on the opposite side, or your cranks won't be parallel to each other.

Pics please

It will work!

 

[Midlife]

BITD the flats needed filing down to get them to fit down so that the correct amount of thread is showing..... convention was the nut is up with the crank pointing backwards (I think)

Shaun

 

[Uncle Monty]

I don't have my little camera at home today, but I will put a picture of the first attempt for your amusement. I thought they should go through the hole. I am going to grind the next pair to size or just until they go in. You dont want them like a dick in a top hat, but It's the tapered flat that does the work of wedging them and I can see how filing that flat would allow the pin to pass further through the assembly. The old ones were a totally random shape with flats filed all over them so I had nothing as a reference. I think I was supplied the wrong size.

 

[jon w]

Prennial Issues with cotter pins.

As said, tap it, tighten it, ride it, tap it, tighten it, ride it... repeat.....

They will draw down eventually. Just make sure the faces of the shaft and pins are clean and flat. If the pin is digging in it will never draw up.

 

[Uncle Monty]

Bit overkill but:


... at least they fit now. Will tinker with fitting them and adjusting the flats this evening.

 

[Jonny69]

They are too big. They shouldn't be a press fit or oversize. It's the taper that locks it in place by forcing it into the curve.

The ones I've had have been a tight fit but you can push them almost all the way home by thumb, then just need tapping in that last bit. The do what jon w said, tap it the last bit, tighten it, ride it, tap it, tighten it, ride it... repeat... Unfortunately most new cotter pins are made of cheese so don't tighten the nut too hard or you'll shear the thread. You want to use the tapping process to lock the taper, not by tightening the nut.

English convention for cotter pin direction is as Shaun says though I normally remember it as crank forwards nut down, American convention is round the other way. Engineering-wise there's little difference, but in practice you'll find that one way round the nuts will catch on your flares and the other way round will not

Anyway, looks like you've got it all under control

 

[Robbied196]

:shock: Its not overkill, its revenge

I'm sure you can make those do the job but they look slightly long as well. I went through a similar thing buying new pins only to find they were either too tight, too long or too short. Then I found a pair I'd had all along in an old tin that probably date from the 60s. They fitted perfectly, it makes me wonder what sizing they use for modern replacement pins :|

 

[Uncle Monty]

Thanks for all the advice. Yes I have pins from two sources and the ones I showed fitted are longer. I have short 3/8 ones as well which I ground down to the same size. I might have to give the Lionels a go - see if I can get that frayed look! I think I have a pair of 70s style denims a la Roger Daltry somewhere. I don't have the hair though...

 

[Iwasgoodonce]

Uncle Monty, Is there a correct direction to put the little smeggers in? Do I put them in opposite sides? If the chainring side crank is at twelve o clock and the cotter bolt is at 9 o clock, should the non drive cotter nut be at 3 o clock?

I haven't had to do a cotter pin in a very long time!