OS #202 super air cleaner each element is pretreated with a special lubricant which traps fine dust particles and keeps them from damaging the engine. This reduces engine wear and promotes longer engine life. This product includes 4 pretreated blue foam filter elements (42mm dia. x 15mm thick). This is primarily for the OS .21 RX-R engine. Change element when it's no longer blue in color (can be as often as every run if dusty).