Package of 500 piecesEPDM rubber (Neoprene) is an extremely durable synthetic rubber membraneInside Diameter .25" | Outside Diameter 5/8" | Thickness .145"Passivated finish with an A70 durometer hardness304 stainless steel for long life corrosion resistanceRubber bonded to stainless steel creates a corrosion resistant washer that cushions bolt and screw heads and seals joints to prevent leaks. The rubber washer provides superior cushioning, sealing and vibration dampening.