The KN204 is the listed KN filter for the 1800C. But the KN303 has the same thread and outside diameter but is a half inch taller. So probably more media inside. Anyone using the 303?
if the thread is the same, and it is simply longer, you can use it. I have used Honda Civic filters for my zx12r (ninja) and now my vtx1300r. The filter is longer.
