If you are replacing your crank arms with shorter cranks this video shows you how.
You can also find this great video at How to Replace Crank Arms at BikeTutor.com.
If you also have to replace the bottom bracket, here’s a great video on how to do that, How to Replace a Cartridge Bottom Bracket at BikeTutor.com.
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Read the complete post at http://ebent.wordpress.com/2008/03/08/how-to-replace-crank-arms/