The O-RING-SET contains 3 pieces of 3.5 in. O.D. standard industry size O-RING use to prevent leaks when customer need to change 3 pre-filter of 10 in. standard under sink reverse osmosis system. Recommend to change all O-RINGs once open the housing. Lubricate with clean lubricant and place in grooves below thread to prevent water leaks. APEC Water Systems does not distribute or sell our products to any 3rd party sellers on Amazon. Please purchase genuine APEC Water system product in “new” condition from Amazon.Com or APEC Water directly for authenticity. Manufacture warranty may not apply in all cases, depending on factors such as use of product, where the product was purchased, or whom you purchased the product from. Please review the warranty carefully and contact APEC Water if you have any question.