Hi mate,
Took the below measurements.
Main muffler portion inlet: at least 85mm
Mid muffler portion inlet: have not measured as cat still attached
Mid muffler portion outlet: at least 85mm
Front pipe inlets: at least 70mm
Front pipe outlets: at least 75mm
Diameters may actually be larger depending from were you measure from, but I measured purely the open area so might be a bit conservative. Hope that helps.