Caps are a heat resistant EPDM Rubber. Can be used for capping hoses, tubing and vacuum lines for testing. Measurements on Caps are the Outside Diameter of the tubing or hose, and not the inside diameter of the Cap. Caps are flexible and can provide a tight seal. Set includes ID sizes: 1/8", 3/16", 1/4", 5/16" and 3/8".