Cotter pins can be loosened and tightened up with a hammer. The crank must be
supported. The big risk is to damage the pin which is made of relatively soft
steel: the threaded end will bend and the threads will be compressed. The other
end will be flattened. Replacement pins come in various sizes and the one you
need will not be in stock unless it is your lucky day. It is easier and safer to
use the special tool for pins: it is made of two handles and and axle; the short
ends are shaped to fit a pin and have an adjustable spacer; the long ends are
for the hands to squeeze hard. After having had bad luck with a hammer, I
purchased this tool, originally for bicycles, from a mail-order bicyle tools
supplier, twenty five years ago. It works very well. I use it also on my first
uni. Try to borrow or purchase one or let the bicycle shop do it for you before
using a hammer which probably will not set your pin tightly enough. I hope this
will help to save many cotter pins.
Michel Y. Roy roym@dgabby.mfldclin.edu