Fit this over your Twin Air filter when your riding in severe conditions such as heavy dust and sand. This coarse foam pre-filter will act as an additional layer of protection and extend the service time between filter cleanings. For Honda CRF 250 R (2004-2009), CRF 250 X (2004-2017), CRF 450 R (2003-2008), CRF 450 X (2005-2017).