Often the cotters don't exactly fit the angle of the flat on the spindle, and a little filing on the flat of the cotter can mate them up better. That is where the squeak usually comes from, crank arm wobbling on the spindle, but you can usually feel it as you go around.

Seems like old cotters were made from some softer metal then the spindle, to conform to it as you drew it up with the nut, or to deform over some "break-in" time as you would snug the nut up after a while.

Crank Cotters used to be 50 cents for a handful, where do you get them now?