Normally, when you install an exhaust gasket for the Yamaha Zuma 125 and use an aftermarket exhaust system, the OEM gasket is usually smaller than the header port for the aftermarket system. Since you can't modify the OEM gasket, you'll need to buy a new one. We have the Yamaha Zuma 125 Large Port Exhaust Gasket available. Its outside diameter is the same as OEM at 37.5mm, but its inside diameter is larger by 5mm. It also uses a more durable material between the copper rings to ensure a long life and better heat resistance to preserve itself from hotter exhaust temperatures.