If you have a Rhino Heavy Duty Crossbar system and want to mount a number of Rhino's kayak and canoe carriers including the S510, S512 or the Kayak Extension Piece S512X you will need the KC-FK8 fitting kit.
Rhino-Rack Kayak Carrier Fitting Kit for Vortex/Heavy Duty Bar (Suits S510/S512/S512X)
Kayak Carrier Fitting Kit; Suite Rhino Heavy Duty Crossbar;
SUITS S510/12/12X-HD SPZ AERO TH/AERO
Kayak Carrier Fitting Kit
Roof Rack Mount Kit