The YBB Rebuild Kit is something you will definitely need at some point in time, depending on how much you ride that YBB bike of yours. The slider is the part that wears out. The rebuild kit comes complete with a new slider, a rubber wiper and the aluminum cup the slider fits into. Replacement spring sold separately & includes elastomer
The Moots YBB Replacement spring can be used to adjust the firmness of your YBB unit. Also, when it's time to rebuild your YBB units, it's a good measure to replace the spring at the same time.
You will need to remove the slider and/or spring for sizing.