This lightweight aluminum cap fits over the stock HPI Baja fuel cap and houses a filter. The filter material will filter the air before it gets to the vent hole in the fuel cap. This prevents the vent hole from getting plugged with dirt, mud, sand, etc. The inside of the aluminum cap is machined to match the taper of the fuel cap, so it locks in place and stays there, yet it's easy to remove for cleaning the filter. Use the included long screw to remove the aluminum cap from the fuel cap for cleaning, use the included short screw to plug the center hole after cleaning and while running.