Ahhh....I see.

The Zero and XI are two different models. The Zero S is about six years old or more. The XI is the "updated" version, but doesn't share many main chassis parts with the Zero. In TQRC's parts section, you'll see parts specific to both models. Only if you see parts labeled as "XI/Zero" are they compatible with each other. Usually, like Tamiya, only suspension parts are compatible, but the rest between the two won't be.