Originally Posted by Matariki

Thanks for this tip. I have an air chisel *** but had never thought to use it for this purpose. How badly, if at all, does it screw up the old cotter?

To the OP - my preferred method is with a bench vise and socket after overnight does of penetrant. I may or may not apply heat depending on my patience level at the moment. Downside to the method is that it's burdensome. I have had some luck with hammers, C-clamps and other homemade presses, but I can say that they have lead to damage (bike or tool) and frustration.

An aside: There is no bike-related job that I enjoy more than filing cotters to fit.