CRF250 18-19 CRF450 17-20 Twin Air Powerflow Kit . The best filter in the world can’t do its job if there are leaks in the seal of the airbox. Twin Air’s PowerFlow Kits ensure a leak-proof, fool-proof fit with a rigid, lightweight, billet-aluminum cage that replaces your flimsy, plastic stock cage, and a billet aluminum flange with a rubber seal that bolts tight to your airbox.