Cotter pins: Why? Why not?

For discussions about bikes and equipment.
Carlton green
Posts: 741
Joined: 22 Jun 2019, 12:27pm

Re: Cotter pins: Why? Why not?

Postby Carlton green » 28 Jun 2020, 2:25pm

I don’t think that it has been mentioned earlier in the thread but the cotter pin’s surface must be flat for it to fit well and it must definitely not have a heigh spot in the middle as otherwise you’re building in a very slight rocking motion into the joint. If filing one these days I’d us a steel straight edge and light source (shining towards me) to check the surface for being flat. If you can identify the extremes of the actual seating area (that’s in contact) then a very very slight concave surface between the two would ensure the best possible functional fit. Well, to my logic that would be correct. I’d want the surface to be as smooth as was possible too, and likewise the axle flats to be perfect and very slightly chamfered.

I wonder if people dry fit the finish filed pins or grease them? My inclination would be to lightly grease for final assembly.

Posts: 943
Joined: 6 Jan 2007, 12:17pm
Location: Clitheroe

Re: Cotter pins: Why? Why not?

Postby jb » 28 Jun 2020, 6:23pm

If you require to knock a threaded shaft out of a bore then it is best to screw a nut on to it, but the shaft should be driven out by a softer metal drift which is in contact with the shaft not the nut. The threads will still spread if the shaft is stuck but it will be contained by the nut and the nut can be screwed off afterwards whilst preserving the thread form. Once there is movement the nut is backed off.
J Bro

Posts: 39484
Joined: 4 Jan 2012, 6:25pm

Re: Cotter pins: Why? Why not?

Postby Brucey » 28 Jun 2020, 6:51pm

FWIW small differences in cotter pin fit and finish are probably largely academic because

a) the cotter pin approaches yield as it is hammered home and
b) the quality of the fit is limited in many (most?) cases by the finish on the BB spindle flat.

this flat is typical in that it has a pretty rough finish

On the latter point the flat is rarely perfectly smooth, such that the cotter pin needs to be hammered home pretty hard (and/or used for a while) before there is really intimate contact between the cotter pin flat and the spindle flat. It is this final movement which can raise a burr on the cotter pin; if the fit is close to perfect (eg because the cotter pin flat and the spindle flat really are smooth and flat) the cotter pin won't have to move far as it is hammered home and this limits the size of any burr that can be raised, regardless of the edge sharpness.

If you tap a cotter pin home (not hard) and then examine the mating face, you will rarely (if ever) see witness marks which show contact over the entire face. If the cotter pin is filed as well as it can be, the usual thing is to see widely spaced marks over the entire face. A well-used cotter pin will be marked over the entire face, and you may even see marks on the cotter pin which (albeit smearily) match the spindle.

If you want the cotter pin to be easier to install, to come out more easily and work better in service, it is probably worth using a small stone on the spindle flats so as to make them properly smooth and flat. Easier said than done of course.