This is a Set of 4 Replacement FOAM elements for the OS #202 Super Air Cleaner (OSMG2384). 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. Includes: 4 pretreated blue foam filter elements (42mm dia. x 15mm thick) Comments:-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). -For Complete Filter: OSMG2384 - NMH 6-2-94 ir/jl