Tight-Fill Top-Seal Caps are installed on top-seal adapters, when not in use, to prevent gasoline vapors from escaping and to prevent water, dust and debris from entering the tank. The OPW 634TT is designed to mate with 4" 633T, 633TE, 61SA, 633TC, and 633TCP Top-Seal adapters. Heavy duty and corrosion resistant, the body is made of Duratuff® to help eliminate rust and oxidation for a long, maintenance-free life. The toggle lever distributes downward pressure to compress its Buna-N gasket evenly, assuring a positive, water and vapor-tight seal. The 634TT can be locked with a padlock or wire seal.