probably a silly question but just bought a 3 inch decat downpipe, centre section and exhaust.. non have come with gaskets, the downpipe to turbo gasket is the same size I know that bit but is it literally just 2 basic 3inch ring gaskets I need to the centre to back box and downpipe to centre? or do I need that donut shaped o ring and the 2 spring bolts? for the downpipe to centre?