5/16" ID x 5/8" OD (about .344" metal ID and .290" rubber ID fits/seals 5/16" fastener) 20 Gauge Hot-Dip Galvanized Steel with Bonded EPDM Rubber Washer. 7500/Box. Price/Box.
ASTM A-527 20 Gauge Hot Dip Galvanized Steel; Black EPDM Rubber, 70 durometer hardness per ASTM D2000; Overall thickness about .130"-.135" before compression; Available compression about 0.030"; EPDM thickness about 0.065".
OPTIONS:
Also available with 3