Originally Posted by
Homebrew01
I used to tap them out gently with a hammer. I would unscrew the nut so that the nut and end of the pin were flush. Then tap with a hammer so it hits both the nut & pin at the same time. That spreads the load around a bit and less likely to mash the end of the pin & threads.
Another precaution is to support the crank arm on something solid so the hammering force isn't absorbed by the frame and bottom bracket shell. A length of 1/2" or 3/4" steel pipe stood vertically under the crank arm so the cotter pin can be driven into its center works well.