"in a few hours I am going over to R's hour so he can hurt me"

So I am going over to Z's house in a few hours so we can play before Thanksgiving dinner with his family.
To put it another way, in a few hours I am going over to Z's hour so he can hurt me.
And we both can get off from his doing that.
I feel a little scared..while Z would never really hurt me...I see how a more hardcore dynamic is entering our sex life.
I like pain, and I like D/s, and I trust Z, so I am willing to do--or talking about someday doing-- all sorts of things that--when practiced by other people--sound too edgy (fisting, candle wax, figging, throat fucking and gagging, breath play, for example).
But is there a line with Z I shouldn't let myself cross?
And how would I know till I got there?