Prevent unwanted particles and debris out of your engine! Filter diameter are measured from inner diameter, to ensure perfect fitment, measure the outside diameter of your carburetor or throttle body.
Steady Garage recommended sizes (some years may vary, best thing to do is measure!):
Yamaha Zuma 125: 39mm
Honda Ruckus/Metropolitan OEM carburetor: 39mm
GY6 CVK 24mm OEM carburetor: 48mm
Honda CB550, CB750, CB900, CB1000, CB1100F: 54mm