Jeff, Be careful if you need a crank they are not necessarily the same if they were not originally for a Cub Cadet. Especially the "free" end (PTO end) may not have the same dimensions,length,diameter,keyway,etc if not from a Cub Cadet. If it is different your PTO clutch won't work. Are you sure it is an internal knock? Have someone crank the engine and someone else listen to locate where the noise seems to be coming from (such as a drive shaft joint). If you are sure it is internal, wait until you look inside and see what else you need and verify that it is worth a rebuild.