3/8" ID x 3/4" OD (about .450" metal ID and .355" rubber ID fits/seals 3/8" fastener) 20 Gauge Electro-Galvanized Steel with Bonded EPDM Rubber Washer. 6000/Box. Price/Box.
ASTM A-527 20 Gauge Hot Dip Galvanized Steel; Black EPDM Rubber, 70 durometer hardness per ASTM D2000; Overall thickness .130" before compression; Available compression about 0.030"; EPDM thickness about 0.065".
OPTIONS:
Also available with backing mater