Package of 500 piecesEPDM rubber (Neoprene) is an extremely durable synthetic rubber membraneInside Diameter 7/32" | Outside Diameter 1/2" | Thickness .14"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.