The K&N RF-1048 Air Filter is a Round Tapered Universal Air Filter. The base OD is 7.5" and tapers down to a 4.5" OD. The overall length of the filter element is 6" (not including the flange). It has a chrome metal top. K&N Air Filters utlize a oiled cotton media, which provide excellent filtration while allowing it to be less dense to improve flow rates. This is the recommended filter for the all the BPI Flow Stacks.