RB Concept gaskets are used to seal the back of .21 to .28 size rear exhaust engines where the tuned pipe manifold attaches to the rear of the engine. These eventually go bad and lose their seal over time due the intense heat of the engine, so it is good idea to replace these when they get worn out or you find they are leaking. This seal is also used for one piece tuned pipe exhaust systems, to seal the connection between the tuned pipe and the manifold.