Instruction of size:1. The inner diameter of 'Inlet' should be equal to the size of your pipe's external diameter.2. 51mm, 57mm, 60mm, 63mm, 67mm, 70mm, and 73mm diameter 'Inlet' are also optional. Please feel free to contact us!Size recommendation:1. Contact us, we will help you with size choosing;2. Outlet size could be 80mm, 93mm, 105mm.3.M2 M3 M4 M5 M6 Original exhaust pipe tip:INLET diameter 70mm (Choose 4 pieces please)4.The car with 'MPerformance Exhaust':INLET diameter 67mm (Choose 4 pieces please)5.M135i M235i M140i M240i M335i M340i M435i M440i:INLET diameter 60mm (Choose 2 pieces please)6. Just in case, you could measure the diameter of your car's exhaust pipe diameter by yourself, and leave us a message about the size you want.Notice:1. The price is only for 1 piece, not for 1 pair. If you want more, please select more.2. If you want an inner shell (IS), please select the outlet 93mm or 105mm.3.The carbon could be gloss or matte, we will deliver gloss carbon by default. If you want matte carbon one, please leave a note or contact us.4. The color could also be blacking or bluing, we will deliver original by default, if you want blacking or bluing you must contact us before ordering!5. Please feel free to contact us, we are willing to solve any problem you meet when place the order！